test-hygiene: DatabaseFixture migration isolation — Story240_ScheduleStore reds under a narrow filter (segment_schedule.show_id missing)

[genwave-radio]

🧪 What

dotnet test tests/GenWave.MediaLibrary.Tests --filter "FullyQualifiedName~Envelope|FullyQualifiedName~Pool" (or any narrow filter that pulls Story240_ScheduleStore in beside a subset of siblings) fails one fact with:

column "show_id" of relation "segment_schedule" does not exist

The same fact is green in the full unfiltered run. Reproduced on main-equivalent trees (git stash of unrelated work) by the T359 reviewer 2026-08-29 — pre-existing, test-order / container-isolation, not a product defect.

🔎 Likely shape

DatabaseFixture applies db/*-migration.sh in a way that depends on which specs share the ephemeral Postgres in a run: under a narrow filter the db/33/db/35 show columns are not present when Story240 executes. The gh-#569 collision lesson (DatabaseFixture) is the nearest precedent.

✅ Done when

• Story240_ScheduleStore is green under any filter that includes it.
• The fixture's migration application is order-independent (or the spec declares its migration dependency explicitly).

Found during the Gardener build loop (T359). P3, test hygiene — pairs with gh-#636.

https://claude.ai/code/session_01AmM47neuvsJph3ZT4brTCV