fix: replace 18 more stack traces with compile errors across codegen

[cs01]

Summary

• When users hit type dispatch errors, class method/field not found, closure capture failures, or unknown variables, they now get clear compile errors with context instead of Node.js stack traces
• 18 throw new Error calls converted to emitError() across 7 files: string-methods.ts, class.ts, map.ts, set.ts, object.ts, variable-allocator.ts, llvm-generator.ts

Test plan

• npm run verify:quick passes (tests + Stage 1 self-hosting)

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