lang: struct consts migrate to const globals, inline fallback (PLAN-CONST-AGG step 4)

A struct constant whose every field serializes — literals, enum tags,
nested aggregates, and (new) const EXPRESSIONS over named consts /
const-aggregate leaves ('r = K + 1', 'g = LIT.r', 'b = A[1]') — becomes
an immutable global: one storage, reads load/GEP it, '@LIT' is
addressable, dead-global elimination drops unused ones. constExprValue
gained a fold-through tail (evalConstIntExpr/evalConstFloatExpr,
source-aware), which also enables const-expression ELEMENTS in array
consts.

A const with a NON-serializable field (a call, a runtime read) keeps
inline re-lowering, and that per-use evaluation is now the documented
contract for the class (pinned: 'CALL.r' reads 1 then 2, side effects
run per use; '#run' is the evaluate-once tool).

Examples: 0180 (migrated shapes + @ptr + copy independence),
0181 (the inline-fallback contract). m3te (23/23) + game rebuilt green.

src/ir/lower.zig

@@ -1610,6 +1610,7 @@ pub const Lowering = struct {
     pub const scanDecls = lower_decl.scanDecls;
     pub const registerTypedModuleConst = lower_decl.registerTypedModuleConst;
     pub const registerConstArrayGlobal = lower_decl.registerConstArrayGlobal;
+    pub const maybeRegisterConstStructGlobal = lower_decl.maybeRegisterConstStructGlobal;
     pub const inferConstArrayType = lower_decl.inferConstArrayType;
     pub const typedConstInitFits = lower_decl.typedConstInitFits;
     pub const constExprInitFits = lower_decl.constExprInitFits;