fix: remove 82 unnecessary as-unknown-as double casts in escape-analysis

[cs01]

Summary

• Removed all 82 as unknown as double-cast patterns in src/semantic/escape-analysis.ts
• Replaced with direct as casts (e.g., stmt as unknown as VariableDeclaration → stmt as VariableDeclaration)
• All casts are from Statement/Expression union types to their constituent members — TypeScript accepts direct casts since the target types are part of the union
• The unknown intermediate bypassed TypeScript's structural compatibility checks and gave the native compiler less type information to work with

Why this matters

• as unknown as is a type erasure pattern that hides bugs from TypeScript — if a type assertion is structurally incompatible, TypeScript can't warn you
• The native compiler uses type information from casts to determine struct layouts for GEP indices — as unknown loses this information
• escape-analysis.ts runs as a semantic pass before codegen and is compiled during self-hosting

Test plan

• TypeScript compiles clean (no type errors)
• All 437 tests pass (node + native)
• npm run verify:quick passes (Stage 0 + Stage 1 + tests)

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