lang: reject writes through constants (PLAN-CONST-AGG step 2, fixes 0116)

Any assignment / compound-assignment whose target chain is ROOTED at a
constant — a const-flagged global (array consts, #run consts) or a
module value const (struct consts incl.) — diagnoses 'cannot assign
through constant X' at compile time. A struct const's field write used
to compile and bus-error at runtime (issue 0116); scalars misfired
silently. A deref along the chain (p.*) breaks the root — pointer
writes stay the documented escape until the const-ness steps; a local
shadowing the const name stays writable.

Also: typed struct constants ('W : Color : Color.{...}') register —
the shape list skipped struct_literal, leaving the typed form
unresolved while the untyped one worked.

Examples: 1162 (all rejection shapes incl. the 0116 crash repro),
0178 (typed struct const reads + copy independence).

src/ir/lower/decl.zig

@@ -801,7 +801,7 @@ pub fn registerTypedModuleConst(self: *Lowering, cd: *const ast.ConstDecl) void
     // foldable / emittable const, so it cannot manifest a
     // wrong-type fold/emit; a use-site diagnostic covers it.
     switch (cd.value.data) {
-        .int_literal, .float_literal, .bool_literal, .string_literal, .undef_literal, .null_literal, .binary_op, .unary_op => {},
+        .int_literal, .float_literal, .bool_literal, .string_literal, .undef_literal, .null_literal, .binary_op, .unary_op, .struct_literal => {},
         else => return,
     }
     const ty = self.resolveType(ta);