feat: add native road control markings

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

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+# Native road control markings
+
+## Goal
+
+Give Nantaizi's native-road provider inspectable, source-traceable crosswalk
+and vehicle stop-line geometry, so intersection control markings do not depend
+on the osm2streets render output.
+
+## Confirmed Facts
+
+- Native road output already owns road surfaces, sidewalks, lane separators,
+  repeated direction arrows, and explicit junction-turn arrows.
+- Nantaizi's OSM input contains explicit marked crossings, including zebra and
+  traffic-signal crossings. The existing osm2streets output currently has 48
+  crosswalk-stripe polygons and 8 stop-line polygons.
+- `build-osm2streets-qgis.js:1067` derives these markings from crossing nodes
+  plus osm2streets driving lanes. Native must not read that rendered geometry;
+  it can reuse only the tested geometry rules after adapting them to native
+  directed lanes and junction plans.
+- Existing Blender materials already provide `crosswalks` and
+  `vehicle_stop_lines`; native may map into them without changing the legacy
+  osm2streets layer registry.
+
+## Requirements
+
+- R1: Native compilation uses explicit marked OSM crossing nodes only, and
+  emits crosswalk-stripe polygons plus approach stop lines only where safe
+  native directed-road placement exists.
+- R2: Each generated feature preserves crossing node, OSM road, native road or
+  lane, direction, placement method, and relevant junction provenance.
+- R3: The Road Workbench independently toggles, selects, and describes native
+  crosswalks and stop lines in Chinese.
+- R4: Native Blender and Cesium builds consume both layers with the existing
+  control-marking materials; no `package/` publication is part of validation.
+- R5: Missing compatible road context, ambiguous geometry, and unsupported
+  crossing inputs are diagnostics; the compiler must not invent a crossing.
+
+## Acceptance Criteria
+
+- [ ] Nantaizi native output contains valid `crosswalks.geojson` and
+  `vehicle_stop_lines.geojson` features with source-traceable properties,
+  without reading osm2streets rendered layers.
+- [ ] A Workbench user can toggle and select either marking type and see the
+  crossing node, associated road/direction, and placement evidence.
+- [ ] Native Blender/Cesium output contains both marking types using existing
+  materials, after `blender,cesium,preview --road-provider native` and without
+  publishing `package/`.
+- [ ] Tests cover a marked crossing, an unmarked crossing skip, a missing or
+  ambiguous native-road placement skip, and output-layer contract checks.
+
+## Out Of Scope
+
+- Hand-placed control-marking overrides, traffic-signal state-machine changes,
+  freehand polygon editing, processing another region, and importing
+  osm2streets-rendered crosswalk geometry.
+
+## Key Decision
+
+The first version is explicit-OSM-only. This is feasible for Nantaizi and
+keeps control markings evidence-backed; missing data remains a diagnostic for
+OSM improvement rather than a silent geometric guess.