feat: compile revisions from area config snapshots

.trellis/spec/backend/road-revision-storage.md

@@ -8,8 +8,10 @@
 
 ```js
 ensureRevisionStore(workspace) // -> { paths, active, baseline }
+setActiveAreaConfig(workspace, options) // -> { paths, areaConfig }
+readAreaConfigSnapshot(file) // -> compiler options
 createCheckpoint(workspace, label) // -> restored revision
-readRevision(workspace, revisionId) // -> { manifest, osm, nativeRoadOverrides, directEdits, trafficSignals }
+readRevision(workspace, revisionId) // -> { manifest, osm, areaConfig, nativeRoadOverrides, directEdits, trafficSignals }
 ```
 
 ## 3. Contracts
@@ -17,14 +19,17 @@ readRevision(workspace, revisionId) // -> { manifest, osm, nativeRoadOverrides,
 - Legacy inputs remain at `source.osm`, `native-road-overrides.json`, and `native-traffic-signals.json`; migration must never rewrite them.
 - OSM bytes are stored once under `osm/<sha256>.osm`.
 - `active/native-road-edits.json` starts as a validated v2 document at version `0`; `active/state.json` points to the active revision.
-- A revision is immutable at `revisions/rev-NNNN/`. Its `manifest.json` uses `road-workbench-revision/v1`, references the content-addressed OSM, lists frozen JSON documents, and records SHA-256 digests.
-- Area-config snapshots are intentionally owned by the subsequent snapshot step. Do not silently fall back to the external config when that step is introduced.
+- The active config is `{ schema: 'road-workbench-area-config/v1', options }` at `active/area-config.snapshot.json`. Updating it must also update `base.areaConfigSha256` through a versioned v2 document write.
+- A revision is immutable at `revisions/rev-NNNN/`. Its `manifest.json` uses `road-workbench-revision/v1`, references the content-addressed OSM and frozen `area-config.snapshot.json`, lists frozen JSON documents, and records SHA-256 digests.
+- `compileInput()` uses `areaConfigSnapshotFile` when present. Its external `options` are ignored in that mode; a missing or invalid snapshot is an error, never a fallback.
 
 ## 4. Validation And Error Matrix
 
 | Condition | Error / behavior |
 | --- | --- |
 | Required legacy input missing | `Revision source is missing: <path>` |
+| Snapshot file is missing | `Area config snapshot is missing: <path>` |
+| Snapshot schema or options are invalid | `Invalid area config snapshot: <path>` |
 | Content-addressed OSM differs from its filename digest | `Content-addressed OSM is corrupt` |
 | Revision manifest schema or ID mismatch | `Invalid revision manifest` |
 | Revision ID is not `rev-NNNN` | Reject with `Invalid revision id` |
@@ -36,7 +41,7 @@ readRevision(workspace, revisionId) // -> { manifest, osm, nativeRoadOverrides,
 
 - Good: first open of a legacy directory adds `osm/`, `active/`, and `revisions/rev-0001/`, leaving old files byte-identical.
 - Base: opening an already migrated directory is idempotent; an interrupted migration with a baseline but no active state repairs only the missing state file.
-- Bad: mutating a frozen revision document or its referenced OSM must make `readRevision()` fail rather than returning altered input.
+- Bad: mutating a frozen revision document, config snapshot, or referenced OSM must make `readRevision()` fail rather than returning altered input.
 
 ## 6. Tests Required
 
@@ -45,9 +50,13 @@ readRevision(workspace, revisionId) // -> { manifest, osm, nativeRoadOverrides,
 - legacy-byte preservation and baseline creation;
 - equal OSM bytes yielding one content-addressed copy;
 - named checkpoint creation, parent linkage, and complete restore;
-- manifest digest equality with the actual frozen files;
+- manifest digest equality with the actual frozen files, including area config;
+- active config writes incrementing `documentVersion` and preserving old revision snapshots;
+- compiler snapshot precedence and an explicit missing-snapshot error;
 - invalid checkpoint labels.
 
+`test/fixtures.js` compares normalized current compiler outputs with the checked-in `native-road-package/v1.1` fixture baselines. Regenerate them only through `npm run test:fixtures:update-baseline` after an intentional compiler output change.
+
 ## 7. Wrong Vs Correct
 
 ### Wrong
@@ -66,3 +75,19 @@ manifest.source = { osmFile: path.relative(paths.workspace, file), osmSha256: di
 ```
 
 The content address is both the deduplication key and the integrity contract.
+
+### Wrong
+
+```js
+compileInput({ ...input, options: externalOptions });
+```
+
+This allows later external config changes to alter a frozen revision.
+
+### Correct
+
+```js
+compileInput({ ...input, areaConfigSnapshotFile: revisionSnapshot });
+```
+
+The compiler validates and consumes the frozen snapshot exclusively.