feat: add OSM turn lane arrows

.trellis/tasks/08-04-add-osm-turn-lane-arrows/research/lane-icons.md

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+# lane-icons research
+
+Source: <https://github.com/openstreetmap/lane-icons>, inspected 2026-08-04.
+
+## Provenance
+
+- Owner: `openstreetmap` GitHub organization.
+- Repository tree revision: `db3cab321da93a757c2cf3cdb4f1f1db6b1a091c` (`master`).
+- License: `LICENSE.md` is CC0 1.0 Universal. Keep that text adjacent to any
+  vendored source assets.
+
+## Relevant Contents
+
+- `docs/through.svg`
+- `docs/left.svg`
+- `docs/left-through.svg`
+- `docs/slight_left.svg`
+- `docs/slight_left-through.svg`
+- `docs/sharp_left.svg`
+- `docs/sharp_left-through.svg`
+- `docs/reverse_left.svg`
+- `docs/reverse_left-through.svg`
+- `docs/reverse_left-left-slight_left-through.svg`
+- `docs/left-slight_left-through.svg`
+
+## Design Rules
+
+`DESIGN.md` bases arrows on the MUTCD Standard Arrow. It specifies a shared
+shaft for compound arrows, smooth curves, aligned arrow bottoms, and reduced
+arrowhead scale as the number of directions grows. The upstream SVGs are
+single-color and stroke-free; our existing render layer supplies the project
+fill and outline treatment.
+
+## Adoption Decision
+
+Vendor all listed arrow SVGs with provenance and CC0 license text. The
+production mapping is initially restricted to `through`, `left`, `right`,
+`through;left`, and `through;right`. Right-side variants are deterministic
+mirrors of upstream left-side assets. Every other vendored arrow remains
+explicitly untested and unavailable to the production mapper.