fix(ir): serialize null pointer fields in global aggregates (issue 0081)

A module-global aggregate initializer rejected a `null` literal in a
pointer (or optional-pointer) field as "must be initialized by a
compile-time constant". `Lowering.constExprValue` had no `.null_literal`
arm, so the null leaf returned no constant and the whole aggregate looked
non-constant — even though `null` is the compile-time zero pointer (a
top-level scalar `p : *s64 = null;` already serialized fine).

Add `.null_literal => .null_val` to constExprValue. While here, make the
two LLVM constant emitters exhaustive: emitConstAggregate and the
top-level init_val switch in emit_llvm.zig previously ended in a silent
`else => LLVMConstNull(...)` catch-all (the silent-arm class CLAUDE.md
mandates rooting out). They now handle every ConstantValue tag explicitly
(.null_val/.zeroinit -> all-zero constant, .undef -> LLVMGetUndef,
.func_ref resolved, nested .vtable is a hard @panic tripwire). The
reject-loud path for genuinely non-constant fields is preserved.

Regression: examples/0138 (array-of-struct null ptr fields, array of
all-null pointers, nested struct-in-struct null ptr) and the negative
examples/1126 (null ptr field beside a non-const field still errors).
Fail-before/pass-after verified.

examples/0138-types-global-aggregate-null-pointer-field.sx

@@ -0,0 +1,48 @@
+// A module-global aggregate initializer may carry `null` in a pointer field:
+// `null` is a compile-time constant (the zero pointer), so the field reads back
+// as null with NO prior store, and its non-pointer neighbors keep their declared
+// values. Covered shapes: an array-of-struct with a null pointer field, a global
+// array of all-null pointers, and a nested struct-in-struct with a null pointer.
+// Regression (issue 0081): the constant-aggregate serializer had no
+// `.null_literal` arm, so a `null` in a pointer field made the whole aggregate
+// look non-constant and the global was rejected with "must be initialized by a
+// compile-time constant". The fix serializes a null literal to a constant zero
+// pointer (the same way a top-level pointer global `p : *s64 = null;` does)
+// while still rejecting genuinely non-constant fields (see diagnostics 1126).
+
+#import "modules/std.sx";
+
+Box :: struct { p: *s64; marker: s64; }
+Inner :: struct { q: *s64; tag: s64; }
+Outer :: struct { inner: Inner; label: s64; }
+
+// array-of-struct with null pointer fields + scalar neighbors
+boxes : [2]Box = .[ .{ p = null, marker = 11 }, .{ p = null, marker = 22 } ];
+// global array of all-null pointers
+ptrs : [3]*s64 = .[ null, null, null ];
+// nested: struct containing a struct with a null pointer field
+nested : [2]Outer = .[
+    .{ inner = .{ q = null, tag = 1 }, label = 100 },
+    .{ inner = .{ q = null, tag = 2 }, label = 200 },
+];
+
+main :: () {
+    print("boxes ptrs={},{} markers={},{}\n",
+        boxes[0].p == null, boxes[1].p == null, boxes[0].marker, boxes[1].marker);
+    print("ptr arr nulls={},{},{}\n", ptrs[0] == null, ptrs[1] == null, ptrs[2] == null);
+    print("nested q nulls={},{} tags={},{} labels={},{}\n",
+        nested[0].inner.q == null, nested[1].inner.q == null,
+        nested[0].inner.tag, nested[1].inner.tag,
+        nested[0].label, nested[1].label);
+
+    if boxes[0].p == null and boxes[1].p == null
+        and boxes[0].marker == 11 and boxes[1].marker == 22
+        and ptrs[0] == null and ptrs[1] == null and ptrs[2] == null
+        and nested[0].inner.q == null and nested[1].inner.q == null
+        and nested[0].inner.tag == 1 and nested[1].inner.tag == 2
+        and nested[0].label == 100 and nested[1].label == 200 {
+        print("PASS\n");
+    } else {
+        print("FAIL: global aggregate null pointer field mis-serialized\n");
+    }
+}

examples/1126-diagnostics-global-aggregate-non-const-field-rejected.sx

@@ -0,0 +1,21 @@
+// A module-global aggregate with a NULL pointer field is fine (null is a
+// compile-time constant), but a sibling field initialized from a NON-constant
+// expression (here a runtime function call) must still be rejected loudly. The
+// presence of an accepted `null` must NOT widen the gate to admit the
+// non-constant neighbor.
+// Regression (issue 0081): the null-pointer fix must not regress the
+// reject-loud behavior for genuinely non-constant initializers (issues
+// 0072/0080). Expected: "global 'boxes' must be initialized by a compile-time
+// constant"; exit 1.
+
+#import "modules/std.sx";
+
+runtime_marker :: () -> s64 { return 7; }
+
+Box :: struct { p: *s64; marker: s64; }
+boxes : [1]Box = .[ .{ p = null, marker = runtime_marker() } ];
+
+main :: () -> s32 {
+    print("marker={}\n", boxes[0].marker);
+    return 0;
+}

examples/expected/0138-types-global-aggregate-null-pointer-field.exit

@@ -0,0 +1 @@
+0

examples/expected/0138-types-global-aggregate-null-pointer-field.stderr

@@ -0,0 +1 @@
+

examples/expected/0138-types-global-aggregate-null-pointer-field.stdout

@@ -0,0 +1,4 @@
+boxes ptrs=true,true markers=11,22
+ptr arr nulls=true,true,true
+nested q nulls=true,true tags=1,2 labels=100,200
+PASS

examples/expected/1126-diagnostics-global-aggregate-non-const-field-rejected.exit

@@ -0,0 +1 @@
+1

examples/expected/1126-diagnostics-global-aggregate-non-const-field-rejected.stderr

@@ -0,0 +1,5 @@
+error: global 'boxes' must be initialized by a compile-time constant
+  --> examples/1126-diagnostics-global-aggregate-non-const-field-rejected.sx:16:18
+   |
+16 | boxes : [1]Box = .[ .{ p = null, marker = runtime_marker() } ];
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

examples/expected/1126-diagnostics-global-aggregate-non-const-field-rejected.stdout

@@ -0,0 +1 @@
+

issues/0081-global-aggregate-null-literal-rejected.md

@@ -0,0 +1,116 @@
+# 0081 - global aggregate null literal rejected as non-constant
+
+> **RESOLVED.**
+> **Root cause:** `Lowering.constExprValue` (`src/ir/lower.zig`) — the
+> constant-aggregate serializer for global initializers — had no
+> `.null_literal` arm. A `null` in a pointer (or optional-pointer) field
+> therefore returned no constant, which propagated up through
+> `constStructLiteral` / `constArrayLiteral` and made the whole aggregate look
+> non-constant, so `globalInitValue` rejected it with "must be initialized by a
+> compile-time constant". A `null` is a compile-time constant (the zero
+> pointer) and a top-level scalar pointer global (`p : *s64 = null;`) already
+> serialized fine — only the nested-aggregate path was wrong.
+> **Fix:** add `.null_literal => .null_val` to `constExprValue` so a null leaf
+> serializes to a constant zero pointer. Made the LLVM constant emitters
+> exhaustive while at it: `emitConstAggregate` and the top-level `init_val`
+> switch in `src/ir/emit_llvm.zig` previously ended in a silent
+> `else => LLVMConstNull(...)` catch-all (the precise silent-arm class CLAUDE.md
+> mandates rooting out); they now handle every `ConstantValue` tag explicitly
+> (`.null_val`/`.zeroinit` → all-zero constant, `.undef` → `LLVMGetUndef`,
+> `.func_ref` resolved, nested `.vtable` is a hard `@panic` tripwire since
+> vtables are top-level-only). The reject-loud path for genuinely non-constant
+> fields (a runtime call, etc.) is preserved.
+> **Regression:** `examples/0138-types-global-aggregate-null-pointer-field.sx`
+> (array-of-struct with null pointer fields, global array of all-null pointers,
+> nested struct-in-struct null pointer — asserts null reads + correct neighbors)
+> and the negative `examples/1126-diagnostics-global-aggregate-non-const-field-rejected.sx`
+> (a null pointer field beside a non-constant field still errors loudly).
+> Verified fail-before (pre-fix rejects 0138) / pass-after.
+
+## Symptom
+
+A module-global aggregate initializer rejects a `null` literal in a pointer
+field as "not a compile-time constant"; expected the null pointer to serialize
+as a constant zero pointer the same way a top-level pointer global does.
+
+## Reproduction
+
+```sx
+#import "modules/std.sx";
+
+Box :: struct {
+    p: *s64;
+    marker: s64;
+}
+
+boxes : [2]Box = .[
+    .{ p = null, marker = 11 },
+    .{ p = null, marker = 22 },
+];
+
+main :: () -> s32 {
+    print("ptrs={} {} markers={} {}\n",
+        boxes[0].p == null,
+        boxes[1].p == null,
+        boxes[0].marker,
+        boxes[1].marker);
+    if boxes[0].p == null and boxes[1].p == null and boxes[0].marker == 11 and boxes[1].marker == 22 {
+        return 0;
+    }
+    return 1;
+}
+```
+
+Observed:
+
+```text
+error: global 'boxes' must be initialized by a compile-time constant
+```
+
+Expected:
+
+```text
+ptrs=true true markers=11 22
+```
+
+## Investigation prompt
+
+Fix issue 0081: module-global aggregate initializers reject `null` literals in
+pointer fields even though `null` is a compile-time constant pointer value.
+
+Suspected area:
+- `src/ir/lower.zig`, `Lowering.constExprValue` — the switch has no
+  `.null_literal` arm, so `constStructLiteral` treats a pointer field initialized
+  with `null` as non-constant and `globalInitValue` reports the whole aggregate.
+- `src/ir/emit_llvm.zig`, top-level `global.init_val` emission and
+  `LLVMEmitter.emitConstAggregate` — both currently rely on catch-all
+  `else => LLVMConstNull(...)` for several `ConstantValue` tags. If `.null_val`
+  is threaded through aggregate constants, add explicit `.null_val` handling
+  there (and explicit `.zeroinit` / `.undef` handling as appropriate) rather
+  than depending on the catch-all.
+
+Likely fix:
+- Add `.null_literal => .null_val` to `constExprValue` for constant aggregate
+  serialization.
+- Ensure LLVM constant emission handles `.null_val` explicitly for both
+  top-level constants and nested aggregate leaves.
+- Keep unsupported aggregate expressions loud: non-constant calls/field-accesses
+  should still diagnose instead of zero-initializing.
+
+Verification:
+- Run the repro above and expect:
+
+```text
+ptrs=true true markers=11 22
+```
+
+- Add a pinned regression in the `01xx` types block covering a global
+  array-of-struct with pointer-null fields (and, if straightforward, optional
+  null fields too).
+- Run:
+
+```sh
+zig build
+zig build test
+bash tests/run_examples.sh
+```

src/ir/emit_llvm.zig

@@ -902,7 +902,12 @@ pub const LLVMEmitter = struct {
                     .string => |sid| self.emitConstStringGlobal(self.ir_mod.types.getString(sid)),
                     .aggregate => |agg| self.emitConstAggregate(agg, llvm_ty),
                     .vtable => c.LLVMConstNull(llvm_ty), // placeholder — initialized in initVtableGlobals after function declarations
-                    else => c.LLVMConstNull(llvm_ty),
+                    // A top-level null-pointer global (`p : *s64 = null;`) and a
+                    // zero-initialized global both emit as the all-zero constant
+                    // of the global's type (issue 0081).
+                    .null_val, .zeroinit => c.LLVMConstNull(llvm_ty),
+                    .undef => c.LLVMGetUndef(llvm_ty),
+                    .func_ref => |fid| self.func_map.get(fid.index()) orelse c.LLVMConstNull(llvm_ty),
                 };
                 c.LLVMSetInitializer(llvm_global, init_val);
             } else {
@@ -2433,7 +2438,13 @@ pub const LLVMEmitter = struct {
                 .string => |sid| self.emitConstStringGlobal(self.ir_mod.types.getString(sid)),
                 .aggregate => |inner| self.emitConstAggregate(inner, elem_ty),
                 .func_ref => |fid| self.func_map.get(fid.index()) orelse c.LLVMConstNull(elem_ty),
-                else => c.LLVMConstNull(elem_ty),
+                // A null pointer field and a zero-initialized field both emit as
+                // the all-zero constant of the leaf type (issue 0081).
+                .null_val, .zeroinit => c.LLVMConstNull(elem_ty),
+                .undef => c.LLVMGetUndef(elem_ty),
+                // Vtable constants are only ever produced for top-level protocol
+                // vtable globals (lower.zig), never as a nested aggregate leaf.
+                .vtable => @panic("nested vtable constant in aggregate is unsupported — vtables are top-level globals only"),
             };
         }
         if (is_struct) {

src/ir/lower.zig

@@ -1033,6 +1033,10 @@ pub const Lowering = struct {
             .float_literal => |fl| .{ .float = fl.value },
             .string_literal => |sl| .{ .string = self.module.types.internString(sl.raw) },
             .undef_literal => .zeroinit,
+            // A `null` in a pointer (or optional-pointer) field is a
+            // compile-time constant: the zero pointer. Without this arm the
+            // aggregate is wrongly rejected as non-constant (issue 0081).
+            .null_literal => .null_val,
             .unary_op => |uo| switch (uo.op) {
                 .negate => switch (uo.operand.data) {
                     .int_literal => |il| .{ .int = -il.value },