add --diagnostics json flag for structured error output

[cs01]

Summary

• Adds --diagnostics json flag to build, run, and ir commands
• On success: outputs {"diagnostics":[],"success":true} to stdout
• On error: outputs {"diagnostics":[{"severity":"error","message":"..."}],"success":false} to stdout
• Normal mode (no flag) is completely unchanged
• Enables reliable agent/tooling integration without regex-parsing error messages

Implementation

• Flag added to both chad-node.ts and chad-native.ts CLI entry points
• In JSON mode, stderr is intercepted during compilation so all error paths (semantic analysis, codegen, semantic passes, parser) produce structured JSON on stdout
• No changes to any files compiled by the native compiler (avoids self-hosting breakage) — only compiler.ts, chad-node.ts, chad-native.ts, and a no-op stub in native-compiler-lib.ts

Test plan

• verify:quick passes (tests + Stage 1 self-hosting)
• Success case: chad build hello.ts --diagnostics json → {"diagnostics":[],"success":true}
• Closure mutation error: structured JSON with error message
• Parser error (object spread): structured JSON with error message
• Normal mode (no flag): output unchanged

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