docs: document direct edit solver and API contracts

.trellis/spec/backend/direct-edit-api.md

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+# Direct Edit Solver And API
+
+## 1. Scope / Trigger
+
+This contract covers the pure direct-edit resolver and the workbench endpoints
+that preview, persist, checkpoint, and rebase `native-road-edits/v2` documents.
+It is required because the feature crosses compiler, storage, and HTTP layers.
+
+## 2. Signatures
+
+```js
+resolveDirectEditConstraints(model, editDocument, context)
+// -> { roadProfiles, junctionPlans, constraintStates, handles, diagnostics }
+
+GET  /api/edit-state
+POST /api/edit-preview
+POST /api/edits
+POST /api/revisions
+POST /api/revisions/:id/rebase
+```
+
+## 3. Contracts
+
+- The resolver is deterministic and must not import `fs`, `path`, `http`, or
+  write files. It accepts a baseline model, an optional validated v2 document,
+  and `{ revisionId, compilerGeometryVersion, previewSeq }` context.
+- `GET /api/edit-state` returns the active document, `documentVersion`, active
+  revision, constraint states, diagnostics, and `road-edit-handles/v1` manifest.
+- `POST /api/edit-preview` accepts `document` (or replacement `constraints`
+  and `operations`) and optional integer `previewSeq`. It returns the same
+  manifest/state data plus compiled layers, echoes `previewSeq`, and never
+  changes active files. Work over the 300ms budget sets `degraded: true`.
+- `POST /api/edits` requires integer `expectedDocumentVersion` and either a
+  complete `document` or replacement arrays. Saves through a staging file and
+  rename, increments `documentVersion`, and returns the active revision.
+- A version mismatch returns HTTP 409 with `{ ok: false, error, current }` and
+  leaves the active document byte-identical.
+- `POST /api/revisions` requires a non-empty `label`; it creates an immutable
+  checkpoint and returns its manifest plus current edit state.
+- `POST /api/revisions/:id/rebase` is read-only and returns status counts plus
+  per-constraint details. `pending`, `conflicted`, and `stale` constraints do
+  not enter geometry solving.
+
+## 4. Validation & Error Matrix
+
+| Condition | Required behavior |
+| --- | --- |
+| Missing or invalid v2 document fields | Reject with a field-specific 400 error |
+| Missing `expectedDocumentVersion` | Reject with 400; do not write |
+| Expected version differs from active version | Return 409 and current version; do not write |
+| Empty checkpoint label | Reject with 400; no revision is created |
+| Missing active workspace for preview | Reject with 400; no files are created |
+| Solver replay anchor cannot resolve | Mark `stale` or `conflicted` with diagnostic; never silently apply |
+| Compiler geometry version differs | Mark all replayable constraints `recheck` without changing values |
+| Invariant violation (lane width, crossing boundaries, self-intersection, connector bounds) | Return a blocking diagnostic; do not clamp or silently repair |
+
+## 5. Good / Base / Bad Cases
+
+- Good: preview the same document later saved with the same active baseline;
+  compiled layers and solver diagnostics agree, while tracked file bytes and
+  mtimes remain unchanged.
+- Base: an empty v2 document yields byte-for-byte baseline geometry and an
+  empty constraint-state list.
+- Bad: two tabs save version `N`; the second save with `N` receives 409 and
+  cannot overwrite the first tab's document.
+
+## 6. Tests Required
+
+- Unit: resolver purity, empty-document identity, five replay statuses, handle
+  manifest fields, and blocking invariant diagnostics.
+- Integration: preview file bytes/mtimes unchanged, `previewSeq` echoed,
+  preview layers matching formal compilation, and `degraded` represented as a
+  boolean.
+- Persistence: atomic save, version-409 no-write path, checkpoint parent and
+  digest integrity, and rebase counts/details.
+
+## 7. Wrong vs Correct
+
+### Wrong
+
+```js
+fs.writeFileSync(activeEdits, JSON.stringify(draft));
+```
+
+This permits half-written documents and allows a stale browser tab to overwrite
+newer edits.
+
+### Correct
+
+```js
+if (expectedDocumentVersion !== current.documentVersion) throw conflict409();
+saveEditDocument(activeEdits, { ...draft, documentVersion: current.documentVersion });
+```
+
+The service checks the version before the atomic version-bumping write, while
+the pure resolver remains reusable by preview and formal compilation.