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

[ChrisonSimtian]

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.