fix: replace inline type assertions with proper ast types in closure-analyzer

[cs01]

Summary

• Replace 20+ local interface definitions in closure-analyzer.ts with proper AST types from ast/types.ts
• Eliminates struct field misalignment risk that caused native compiler SIGSEGV on describe.ts (issue handler registry #22)
• Reduces file from 449 to 293 lines by removing redundant type definitions

Test plan

• npm run verify:quick passes (tests + Stage 1 self-hosting)
• TypeScript type check passes
• describe.ts test remains @test-skip — needs native compiler rebuild to verify fix

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