fix: top-level switch statements silently dropped

[cs01]

Summary

• switch statements at the top level (outside functions) were silently dropped by both parsers — no error, just ignored
• Both parser-ts and parser-native now handle top-level switch by transforming to if-else chains (same as inside functions)
• Also discovered that braces in switch cases cause the switch-to-if-else transform to treat them as fall-through (Block nodes return null from transformStatement) — used brace-free case syntax to match existing patterns

Test plan

• New test fixture control-flow/switch-string-toplevel.ts
• All existing tests pass
• Self-hosting (Stage 0 + Stage 1) passes
• Native compiler correctly compiles top-level switch

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