feat: add native road compiler workbench

.trellis/tasks/08-13-native-road-compiler/prd.md

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+# Native road compiler workbench
+
+## Goal
+
+Build an incremental native road compiler for Chinese urban and campus OSM
+data that can progressively exceed osm2streets in geometry quality,
+explainability, and repeatable correction. The existing osm2streets pipeline
+must remain usable while the native compiler is developed and compared.
+
+The first deliverable is a browser-based Road Workbench. It must expose the
+native compiler's source data, generated geometry, and diagnostics, allow
+users to make small semantic/topology corrections, persist those corrections
+as versionable overrides, and reload them automatically in later runs.
+
+## Confirmed Facts
+
+- Nantaizi currently works relatively well because its OSM data received
+  deliberate supplemental tagging; it still has missing boundaries and
+  polygons that cannot be closed.
+- Other tested OSM inputs expose osm2streets sensitivity to input structure
+  and leave too much opaque, final-polygon repair work in QGIS.
+- Existing Blender consumes the nine GeoJSON render layers from
+  `osm2streets_web_out/`; QGIS GeoPackage edits can currently be reimported
+  only as a whole batch.
+- The repository has no existing interactive browser editing service. Existing
+  Cesium preview is a static, generated verification page.
+
+## Requirements
+
+- R1: Add a native-road-compiler path without replacing or regressing the
+  existing osm2streets path.
+- R2: Parse OSM into a canonical, source-traceable road model with stable
+  references to OSM ways and nodes, explicit values versus inferred values,
+  and diagnostics.
+- R3: Compile at least ordinary road segments and the initial supported
+  junction subset into the existing render-layer contract, allowing existing
+  Blender/Cesium consumers to be reused.
+- R4: Provide a browser Road Workbench that overlays raw OSM topology,
+  generated geometry, osm2streets comparison geometry when available, and
+  compiler diagnostics.
+- R5: The workbench must permit scoped user adjustments and save them to an
+  area-local, human-reviewable override file. Future compile and workbench
+  runs must load that file automatically.
+- R6: Each generated object and diagnostic must be traceable to OSM source
+  IDs, compiler rule/inference evidence, and relevant override IDs.
+- R7: Validate topology and geometry before publishing generated layers;
+  report unresolved semantic errors instead of silently disguising them as
+  geometric repair.
+- R8: Develop against nantaizi plus problem inputs and report native versus
+  osm2streets comparison metrics.
+
+## Scope Boundaries
+
+- First implementation targets Chinese urban/campus roads, ordinary road
+  segments, T/cross junctions, directed/multi-lane roads, and data already
+  tagged in nantaizi where possible.
+- Existing Blender, Cesium export, package format, building, vegetation, and
+  water generators are out of scope unless a compatibility adapter requires a
+  narrowly scoped change.
+- Directly editing final render polygons is not the intended correction model;
+  generated layers remain derived output.
+- Complex interchanges, arbitrary multilayer junctions, and full worldwide OSM
+  coverage are deferred until driven by concrete samples.
+
+## Acceptance Criteria
+
+- [ ] A native compile command produces a canonical road model, generated
+  layers, diagnostics, and comparison artifacts for a configured area without
+  changing the osm2streets output path.
+- [ ] A browser command serves a Road Workbench for an area and clearly shows
+  source topology, generated output, diagnostics, provenance, and saved
+  overrides.
+- [ ] A user can make the agreed first-scope override edits in the browser,
+  save them explicitly, and receive a durable area-local override artifact.
+- [ ] Re-running compile or reopening the workbench applies saved overrides
+  automatically and exposes their provenance.
+- [ ] The compiler reports invalid/unclosed geometry, dangling road ends,
+  and unresolved junction/lane ambiguity with source IDs.
+- [ ] Nantaizi and at least one known problematic area can run through the
+  native analysis/preview path, with comparison metrics captured rather than
+  a claim based only on visual inspection.
+
+## Key Decisions
+
+- The first browser editing surface supports road parameters (width, directed
+  lane counts, left/right sidewalk state) plus junction endpoint
+  connect/disconnect decisions.
+- Turn restrictions, stop lines, and crosswalk placement are deferred until
+  the compiler has a validated road/junction editing loop.
+- Overrides are a versioned, human-reviewable JSON artifact owned by the area,
+  not edits to generated polygon layers.
+- Native output and osm2streets output remain parallel during development;
+  neither silently overwrites the other.