feat: add native road control markings

.trellis/tasks/08-17-native-road-control-markings/design.md

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+# Native Road Control Markings Design
+
+## Architecture
+
+The native compiler remains the source of truth. It extends its canonical OSM
+parse with marked crossing nodes, then derives two additive polygon layers from
+the crossing evidence, native directed lanes, and native junction plans:
+
+```text
+OSM crossing node + native directed lane centerlines + junction plans
+    |
+    +-- crosswalks.geojson: six zebra stripe polygons per safe crossing
+    |
+    +-- vehicle_stop_lines.geojson: one safe approach stop-line polygon
+```
+
+No osm2streets GeoJSON is read. Existing crossing geometry helpers may be
+extracted or adapted only when they operate on native lane data and retain
+native provenance.
+
+## Source And Placement
+
+- A source node is eligible only when `highway=crossing` and
+  `crossing:markings` is not `no`, `none`, or `unmarked`.
+- The compiler finds native roads containing the crossing's OSM node and uses
+  the nearest compatible directed lane centerline to obtain the road tangent.
+- Crosswalk stripes are perpendicular to that tangent and constrained to the
+  native road width. Existing fixed zebra dimensions are retained initially:
+  six 0.45m stripes with 0.45m gaps, 0.45m stripe width, and a highway-based
+  stripe length.
+- A stop line is generated only when the crossing can be associated with a
+  supported junction approach and a safe outside-of-junction side. Otherwise
+  the crosswalk may remain valid but the missing stop line is diagnostic.
+- Duplicate nearby crossing nodes use a stable cluster representative so one
+  physical crosswalk does not produce duplicate stripes.
+
+## Contracts
+
+`layers/crosswalks.geojson` and `layers/vehicle_stop_lines.geojson` are Polygon
+FeatureCollections. Each feature records its crossing OSM node, source OSM way,
+native directed road/lane when available, direction, placement method, and
+`native-road-crosswalk/v1` or `native-road-stop-line/v1` provenance.
+
+The Workbench API returns both layers as `state.layers.crosswalks` and
+`state.layers.vehicleStopLines`. Its browser map uses separate toggleable
+layers and selection evidence; the scene-preview toggle leaves real control
+markings visible.
+
+`catalog.NATIVE_ROAD_LAYERS` maps the sources to existing `crosswalks` and
+`vehicle_stop_lines` material layers. This native adapter must not add them to
+the osm2streets `ROAD_LAYERS` / `SCENE_LAYERS` registry.
+
+## Compatibility And Rollback
+
+The new files are additive under `native-road/layers/`. Existing osm2streets
+output and QGIS input are unchanged. Selecting `--road-provider osm2streets`
+remains rollback. A native Blender build treats a missing new layer as an error
+rather than silently omitting a visible marking.
+
+## Risks
+
+- Crossing nodes can be detached from a routable road or lie on an ambiguous
+  multi-road segment. These become diagnostics rather than guessed geometry.
+- A physical crossing split into several OSM nodes must deduplicate stably.
+- A crosswalk near a non-supported junction may get stripes but no valid stop
+  line; this difference must be exposed in workbench provenance.