test(data): document composition→derive pattern for resource reads

[krisnye]

Adds a db.derive unit test documenting the migration from Observe composition to db.derive for resource reads.

// before
const sum = Observe.withMap(
  Observe.fromProperties({ x: db.observe.resources.x, y: db.observe.resources.y }),
  ({ x, y }) => x + y,
);

// after
const sum = db.derive(db => db.resources.x + db.resources.y);

The test asserts the derive:

• emits the initial sum on subscribe, and
• recomputes when either resource changes (both reads are auto-tracked — no hand-listed inputs).

The callback param is intentionally named db to shadow the outer db, so a read cannot bind the outer (full) db and silently escape dependency tracking.

Test-only; no version bump.

🤖 Generated with Claude Code