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.

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 },