chore(task): archive OSM turn lane arrows

.trellis/tasks/archive/2026-08/08-04-add-osm-turn-lane-arrows/implement.md

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+# Implementation Plan: OSM turn-lane arrows
+
+## Steps
+
+1. Add the complete lane-icons arrow asset set, upstream revision metadata,
+   CC0 license text, and a manifest with the five supported/tested entries and
+   explicit untested status for all other entries.
+2. Add a small CommonJS helper for asset-manifest loading, source-SVG path and
+   stroke conversion, maneuver parsing, road-local placement, and diagnostic
+   skip records. Keep it dependency-free and separately testable.
+3. Wire the helper into the intermediates build so custom features are merged
+   into `laneArrows` before the existing GeoJSON write and normalization path.
+4. Add Node tests for the five supported maneuvers, forward/backward endpoint
+   choice, lane index placement, deterministic order/id, and every required
+   skip reason. Assert that untested manifest entries cannot be selected.
+5. Generate `right` and `through;right` inspection samples from the exact
+   local templates and request user confirmation. Do not enable custom output
+   until this gate is met.
+6. Run the focused tests, syntax checks, the Nantaizi intermediates build, the
+   full Nantaizi chain, and `check:area`. Compare against a turn-tag-removed
+   control to prove an observable GeoJSON difference.
+
+## Validation
+
+- `node --check` for changed CommonJS scripts.
+- New pure Node test script(s), invoked directly or through a package script.
+- `npm run build:area -- --config config/areas/nantaizi-lake-innovation-valley.json --stages intermediates`
+- Full target-area pipeline after visual approval.
+- `npm run check:area -- --config config/areas/nantaizi-lake-innovation-valley.json`
+
+## Review Gates
+
+- No generated custom feature may use an asset whose manifest status is not
+  both supported and tested.
+- Samples must show the same fill/outline treatment as the existing lane-arrow
+  layer.
+- Keep the existing layer registry unchanged; this is a new source for an
+  existing layer, not a new layer.
+- Do not commit without the user's explicit confirmation.