fix: emit compile error for global-scope map instead of segfaulting

[cs01]

Problem

const m = new Map<string, string>(); m.set("k", "v"); at the top level (global scope) segfaults at runtime. This is because global maps are stored as global %StringMap* null (pointer-to-pointer), but m.set() treats them as direct struct pointers (like local alloca %StringMap). The calling convention mismatch causes memory corruption.

Maps inside functions work fine. Sets at global scope also work fine (different allocation path).

Fix

Emit a clear compile error instead of silently generating wrong code:

error: Map operations at the top level are not supported — wrap in a function
  = help: function main() { const m = new Map(...); ... } main();

This is the safe approach — a proper fix would require changing how global map variables are stored and loaded (using an indirection layer), which is a larger change.

Test plan

• New fixture: map-global-scope-error.ts (expects compile error)
• All 460 tests pass