fix: runtime error for json.parse invalid input (#295)

[cs01]

Summary

• JSON.parse<number[]> with invalid input now prints a clear error message and exits instead of returning a null pointer (which caused segfaults on subsequent array access)
• JSON.parse (typed object parse) retains safe behavior: returns default-initialized struct on invalid JSON (empty strings, 0 for numbers)
• Untyped JSON.parse already had error handling (no change)
• Adds test fixture for invalid JSON input

Part of fallible C bridges work (#41 in future-work.md).

Test plan

• npm run verify:quick passes
• New test json-parse-invalid.ts exits with code 1 and error message
• Existing json-safe-parse-test.ts still passes (typed parse returns defaults)

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