fix: implement variable shadowing in if/else blocks

[cs01]

Summary

• let x = 2 inside an if block no longer overwrites an outer let x = 1 — the outer variable is correctly restored after the block exits
• Adds pushScope/popScope calls around if/else block generation in control-flow.ts
• Symbol table saves and restores variable metadata (alloca, type, kind) when a name is redefined in a scope
• Uses primitive arrays (strings, numbers) for saved state to avoid native compiler issues with object-in-array patterns

Test plan

• New test fixture control-flow/variable-shadowing.ts — tests if/else shadowing with numbers, strings, and triple nesting
• Updated symbol table unit tests for new shadow/restore behavior
• All 546 tests pass (JS + native compilers)
• Self-hosting Stage 1 passes

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