Build succeeds. The change in foundations/src/panic/hook.rs:

• Added use std::io::Write; for the writeln! macro.
• Replaced eprintln!("{}", json_output) with acquiring the locked stderr handle and using let _ = writeln!(...) to discard any write error.

This prevents a double-panic: previously, if stderr was closed, eprintln! would panic inside the panic hook. Now the write error is silently ignored as suggested in the issue.

Closes #224

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