fix: ts parser populates topLevelItemTypes, codegen handles top-level switch

[cs01]

Summary

• TS parser was never populating topLevelItemTypes array, causing it to be empty while topLevelItems had entries. This could cause type mismatches when getTopLevelItemType() fell through to inspecting item.type
• Added topLevelItemTypes pushes for all top-level statement types (variable declarations, expressions, assignments, for/for-of/while/do-while/if/try/switch)
• Added "switch" case to processTopLevelItem() in codegen — previously a top-level switch would fall through to generateExpression() which doesn't handle statements

Test plan

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