Design: OSM turn-lane arrows

Boundaries

scripts/build-osm2streets-qgis.js remains the intermediates owner. It will append generated custom features to the existing in-memory laneArrows collection before it is written as lane_arrows_webscale.geojson.
No render layer, QGIS symbol, Blender material, or Cesium export contract changes. Existing lane_arrows_webscale styling and catalog ownership apply to generated features automatically.
Vendored upstream artwork lives under a dedicated repository-local asset directory with its CC0 license and a machine-readable manifest. The manifest is the source of truth for upstream path, source revision, local path, support status, and test status.

Import every arrow SVG from openstreetmap/lane-icons/docs/ that represents an arrow. Preserve the source bytes and retain LICENSE.md with the assets.
Mark only through, left, right, through;left, and through;right as supported and tested in the manifest. All remaining imported arrows are unsupported and untested; no production parser may select them.
The upstream repository has left-side examples only. Derive right and through;right through a deterministic horizontal mirror of the matching left-side geometry, recording derived_from in the manifest.

Parse OSM ways once using the existing OSM parser.
Read turn:lanes:forward / turn:lanes:backward, split lane declarations by |, and map only the five supported normalized maneuvers to manifest entries.
For each valid lane near a determinable intersection endpoint, calculate a stable lane center and road-local axis; transform the selected local arrow polygon into WGS84 coordinates.
Add a GeoJSON feature with existing render-layer behavior and explicit provenance: custom source, OSM way id, direction, lane index, maneuver, source asset, and deterministic feature id/order.
Record a structured skipped item when tags, lane geometry, or endpoint classification are insufficient. Do not invent a fallback location.
Existing normalization, scene merge, GeoPackage import, Blender assembly, and Cesium export consume the expanded collection unchanged.

Parse only the vendored SVG subset used by supported assets at build time: line elements and path commands are converted into local meter-scale polygons, with cubic curves sampled and SVG strokes expanded into outlines. The source SVG is the sole geometry authority; no hand-drawn replacement template is permitted. This stays dependency-free because the supported upstream syntax is intentionally small and covered by Node tests.
Use the upstream Standard Arrow shape as the geometry basis, but let the existing QGIS fill and outline preserve the project's visual language.
Preserve the existing osm2streets-generated arrow features. Custom features are additive and have provenance that distinguishes them from upstream osm2streets output.
Deterministic ordering is by way id, travel direction, and lane index.
Each asset manifest entry defines the source SVG's shaft-axis anchor_x. Placement aligns that axis, rather than the SVG viewBox center, with the lane centerline. Mirrored right-side variants mirror around the same axis.
Upstream SVG units are converted at 0.10 m per unit. This calibrates their local extent against the existing approximately 1.4 m osm2streets arrows; do not use SVG display-pixel scale as road-marking scale.

OSM lane counts, one-way semantics, or endpoint topology can be incomplete. These cases remain skipped with diagnostics rather than creating misleading markings.
Asset import is self-contained and CC0. Rollback removes the custom feature generation and local asset directory; the existing arrow layer remains unchanged.
User approval of the two initial visual samples is required before the feature is enabled in production output.