fix: compile-time errors for silent map/set method dispatch fallthroughs

[cs01]

Summary

• Non-string Set fields (this.fieldName.add/has/delete()) silently fell through with no IR emitted and no error when thisFieldSetValueType !== "string" — now dispatches to numeric set handlers
• String Map/Set local variables with missing alloca silently fell through to wrong-type handlers — now emits compile-time error
• All three paths previously produced broken binaries with no diagnostic

Test plan

• npm run verify:quick passes (tests + self-hosting Stage 0 + Stage 1)
• No new test failures

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