fix: replace 16 stack traces with compile errors for property/variable access failures

[cs01]

Summary

• When users access a non-existent property on an object, class, or interface, they now get a clear compile error with file/line info instead of an ugly Node.js stack trace
• Same improvement for index access errors (wrong key type, unsupported assignment targets) and unary operator errors (increment/decrement on undeclared variables)
• 16 throw new Error calls converted to emitError() across 4 files: member.ts, index.ts, struct-access.ts, unary.ts

Test plan

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

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