fix: support Map<string, number> with value boxing/unboxing

[cs01]

Summary

• Map<string, number> was completely broken — stored numeric values as raw i64/double into i8* slots, causing LLVM type errors or wrong values at runtime
• Added value boxing in generateStringMapSet: converts double/i64/i1 → double → bitcast to i64 → inttoptr to i8*
• Added value unboxing in method-calls.ts get path: ptrtoint i8* → i64 → bitcast to double when mapMeta.valueType === "number"
• Fixed global StringMap allocation to extract value type from MapNode (was hardcoded to "string")
• Added 3 new test fixtures: map-methods (has/delete/clear/size), map-string-keys (Map<string,string>), map-string-number (Map<string,number>)

Test plan

• 446/447 tests pass (1 pre-existing promise-race SIGKILL)
• Self-hosting chain passes (quick mode)
• New map-string-number fixture verifies set/get/has/delete/size all work correctly