docs(m3): place create decisions in the contracts

- DATA-MODEL.md: storage engine (stdlib sqlite3 + schema.sql, no ORM/migrations),
  pragmas, per-test isolation.
- API.md: revision coupling, async job model, PUT check order, server-derived ready.
- PROMPTS.md: no prompt files ship; the DB is the only prompt source.

docs/contracts/API.md

@@ -36,7 +36,12 @@ The outbound WooCommerce payload is owned by `docs/contracts/WOO-API.md`.
 - **Envelope:** every error is `{ "error": { "code": string, "message": string, "detail": object|null } }`.
 - **Lists:** returned in full, no pagination.
 - **Revisions:** create/update carry `client_rev`; the server echoes `server_rev`; a mismatch is
-  `409 revision_conflict`.
+  `409 revision_conflict`. A background inference write bumps `server_rev`; the client adopts the
+  polled `server_rev` before its next write.
+- **Async work:** inference and publish run through one in-process job queue with a single
+  consumer; a status transition is compare-and-set inside one write transaction, so a poll never
+  observes a half-applied transition. Tests run the queue inline so contract tests are
+  deterministic.
 - **IDs and time:** slab ids are client-generated UUIDv4; timestamps are UTC ISO-8601.
 - **Photos:** `multipart/form-data` with `data` (JSON), `files[]` (original images), and `meta[]`
   (`{ "seq": n }`). One photo is the calibration photo, chosen by the owner.
@@ -108,8 +113,10 @@ completion.
 
 ### `PUT /api/v1/slabs/{id}`
 Partial update of editable fields plus `client_rev`. Stores client-sent derived fields and does
-not recompute them. `409 revision_conflict`, `409 duplicate_sku` (SKU field actions `edit_sku` /
-`open_existing`), `422 stale_field` (current options included).
+not recompute them. Checks run in order: `409 revision_conflict`, then `409 duplicate_sku` (SKU
+field actions `edit_sku` / `open_existing`), then `422 stale_field` (current options included).
+`status` becomes `ready` only when the full required set is present; it is server-derived and a
+client never sets it.
 
 ### `DELETE /api/v1/slabs/{id}`
 Soft delete of unpublished drafts only (sets `deleted_at`); `204`. Deleted rows are excluded from