Test layers: in-memory and SQLite-backed Database #76

Signal

We want official test layers:

1. Db.layerSqliteMemory(tables): real SQL on in-memory SQLite, schema created from the table definitions. It is fast and runs in Bun.
2. A fake Database that records the compiled statements and returns given rows, for pure unit tests.

Why (MyRemotely)

• A rule in our codebase: new tests use no module mocks. Seams are replaced by Effect Layers.
• design/gap-review.md lists "a mock Database for users' tests" as P1.
• Portable table definitions (see that issue) make the SQLite layer a real stand-in for D1 logic.

Part of #52: the CAFE-stack roadmap signal from MyRemotely. This shows the direction we want, and it is not a demand. We can send a PR if you accept the idea.