[Foundation] FT-9: Reentrancy/isolation test harness

## Summary

Add a reentrancy/isolation test harness that locks in the de-statification invariant and guards against regressions.

## Scope

- A test that runs two distinct builds back-to-back in one process and asserts full isolation:
- independent parameters/args,
- independent log output (no carry-over),
- independent tool-path config,
- no accumulated event handlers.

## Acceptance criteria

- The two-builds-in-one-process test passes and fails loudly if any per-run state leaks.

## Notes

- **Non-breaking** (test only).
- Depends on: FT-1, FT-2, FT-3, FT-4, FT-5.
- Size: **S**.