Chart features: buoys, beacons and other seamarks on the map (and optionally in routing)

[DocGerd]

User request (2026-07-15): include buoys and other markings on the map, if available.

Scope (future phase, after Phase F)

• Render OSM seamark:* features (buoys, beacons, leading lines, fairway boundaries, harbour features) as a map overlay in the planning UI — data is available in OSM for the whole planning area and fits the offline PMTiles pipeline (own extract or overlay tile layer).
• Optional stretch: use seamark:fairway polygons and dredged-depth tags as a supplementary navigability source in the mask pipeline where EMODnet's DTM is too coarse for dredged channels (directly complements the fix for Mask: 14 of 44 harbors disconnected from open water at default safety depth (incl. Flensburg) #6).
• Strictly a planning aid: seamark rendering must carry the existing "no chart authority" disclaimer; no claim of completeness.

Notes

• Candidate source: OSM seamark tagging (seamark:type=buoy_lateral, buoy_cardinal, beacon_*, light_*), regionally well-mapped in the Flensburg Fjord / Danish South Sea area.
• Keep pipeline build-time only (no runtime fetches), consistent with the existing offline design.

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