[scalvert]

Stacked on #358.

Problem

The rule for keeping a Project on disk at teardown was split across three places. createBintastic marked the Project preserved after a Debug mode Run and logged its path. The Project lifecycle read BINTASTIC_DEBUG again at teardown. The lifecycle also exposed preserve() and active only so that createBintastic could reach in. Understanding Debug mode meant reading all three.

Change

• Merge project-lifecycle.ts into createBintastic. One module now owns the active Project, its preserved flag, starting Runs, and teardown. planRun stays the pure seam for args, binPath, and Node flags.
• The rule is now written down in CONTEXT.md as Preserved Project: a Project is kept when any Run against it was in Debug mode, or when BINTASTIC_DEBUG is on in the process env at teardown. A Run's own env can turn Debug mode on for that Run, which also preserves its Project, but it can't stop the Project being preserved. This matches the behavior on Keep preserved fixtures across the suite and make teardown safe #358. The debugging and usage guides now say so too.
• "Debugging enabled. Fixture: …" is logged once per Project, at its first Debug mode Run, not on every Run. This is the only visible behavior change.

Tests

The tests now run against the public interface, with one test per behavior.

• create-bintastic-test.ts replaces project-lifecycle-test.ts and the createBintastic block of index-test.ts. Fake Projects come in through createProject, and env changes use vi.stubEnv.
• Cases that run-plan-test.ts already covers (binPath variants, static args) are no longer repeated end to end.
• The json, text, and BintasticProject tests move to their own files unchanged.
• New: a test for "a Run's own env can't stop its Project being preserved", a test for the log-once behavior, and a test for setupProject right after a Debug mode Run.

Verification

• npm test passes: lint, format, docs check, and 73 tests.
• Self-review with six lenses on two model families found 3 smaller items. All are fixed in "fix: address self-review findings".
• npm run build and npm run test:package pass.
• Mutation check: removing the teardown env check, or the log-once guard, fails 4 tests in create-bintastic-test.ts.