nit(canvas): canvas-roundtrip.test.ts afterEach posture diverges from file-db-roundtrip precedent

[utof]

From v0.4 Plan 1 Task 6 code review (commit 2e918a5).

1. The test closes db in afterEach with a module-scoped reassigned handle; the repo precedent (tests/integration/file-db-roundtrip.test.ts:37-39, save-overlay, reconciler-crash) keeps db strictly test-local and only rmSyncs in afterEach. Fine for a single test, fragile if the describe grows.
2. beforeEach inlines literal 'n1'/'s1' seed SQL instead of a parameterized seedNote(id) helper like layouts.test.ts — duplication trap when a second scenario lands.

Both plan-level nits (verbatim transcription); restructure when the file next grows. Timestamp-durability assert already fixed inline (5b1cd92).