fix: use correct struct type for stringmap .size dispatch

[cs01]

Summary

• .size on Map<string, *> variables now dispatches to generateStringMapSize (uses %StringMap*) instead of generateMapSize (uses %Map*). Same layout so it worked by luck, but technically wrong LLVM IR type.
• Same fix for this.field.size on Map<string, *> class fields
• Added generateStringMapSize to IStringMapGenerator interface
• Added stringMapGen to MemberAccessGeneratorContext for property handler access
• Removed no-op try { ... } catch (e) { throw e; } in response method dispatch

Test plan

• All 448 tests pass
• Self-hosting passes (quick mode)
• Existing map-string-keys.ts test covers .size on StringMap

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