fix: map<string, number>.set() and charat/charcodeat default args

[cs01]

Summary

• Map<string, number>.set() regression fix: The native compiler miscompiled the charAt(0) numeric literal detection in generateStringMapSet, causing store i8* 0.5 (invalid IR) instead of boxing the double. Fix: pass the declared value type ("number") from dispatchStringMapMethod through to generateStringMapSet, bypassing string-based detection entirely.
• charAt()/charCodeAt() default args: Per JS spec, both methods default to index 0 when called with no arguments. Previously they required exactly 1 argument.

Test plan

• map-string-number-param.ts now passes with native compiler (was failing with invalid IR)
• New string-charat-default.ts fixture tests 0-arg and 1-arg forms
• npm run verify:quick passes (all tests + Stage 1 self-hosting)

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