e2e: assert teardown coverage (no leaked seed/rows after render, preview, and failed render)

[jbrecht]

Problem (gap)

The teardown lifecycle is the busiest area of the post-0.9.0 backlog (#15, #16, #17, #21) yet packages/example-app/test/e2e.ts has no assertion that teardown actually ran to completion. The #8 e2e exercises that onTeardown thunks fire, but nothing fails the build if a future change reintroduces a leak on the setup-failure path (#15) or the preview path (#16).

Per MEMORY.md, e2e.ts is the integration gate and the project uses a verify-then-fold workflow — leaks are exactly the class of silent regression that gate should catch.

User story

As the maintainer, I want the e2e suite to fail loudly when a render, a preview, or a failed render leaves seeded data behind, so the teardown fixes in #15/#16 can't silently regress.

Acceptance criteria

• An adapter/tutorial in the e2e app seeds a countable fixture (e.g. a row) in setup and tears it down in teardown.
• e2e asserts the count returns to baseline after: (a) a successful full render, (b) a previewStep invocation (once preview leaks the adapter seed: runs adapter.setup but not adapter.teardown #16 decides preview's teardown policy), (c) a render whose step throws.
• The setup-failure path (Setup-phase failures (adapter.setup / tutorial.setup) skip teardown → seeded data leaks #15) is covered: a tutorial whose setup throws after seeding must leave baseline count.

Depends on the teardown-policy decisions in #15 and #16. File now so the fixes land with a regression gate.

[jbrecht]

Resolved in PR #25 (released 0.10.0). See the CHANGELOG entry for 0.10.0.