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 @@
+