Load bus stops in chunks so long routes stop timing out

[dizzafizza]

Summary

Bus stops weren't loading (or took too long) on longer routes. The corridor was sent to Overpass as one all-or-nothing query that embedded the coordinate string four times (roads + three separate bus-stop tag clauses). On long routes that regularly exceeded the 25s server timeout, and because the failure was swallowed, every bus stop and speed limit was silently lost. The 700-point corridor cap also widened point spacing past the 90m search radius on very long routes, cutting corners past stops even when the query did succeed.

Changes

• Split the route corridor into independent chunks (~12km each, two in flight at a time) instead of one mega-query — each chunk is small and fast enough to finish well inside its own timeout.
• Bus stops now stream onto the map progressively as each chunk lands (onPartialBusStops callback), instead of only appearing after the whole route finishes processing.
• A failed chunk only loses its own stretch of road/stops instead of the entire route; failures are logged instead of silently swallowed.
• Collapsed the three separate bus-stop tag clauses (highway=bus_stop, public_transport=platform, public_transport=stop_position) into a single regex clause — one corridor pass server-side instead of three, with exact filtering kept client-side in tagsDescribeBusStop.
• Deduplicated results across chunk seams by OSM element id, and merged bus stops that are mapped twice a few meters apart (platform + stop_position nodes) so markers and dwell stops aren't doubled.
• Corridor spacing now stays within the search radius up to ~285km of route (24 chunks × 160 points) instead of degrading past ~50km.

Testing

• Verified the exact generated query shape against the live Overpass API: HTTP 200 in ~2s, response fields (element ids, way geometry, node lat/lon) match the Swift decoder's assumptions, and the collapsed regex clause correctly returns both legacy (highway=bus_stop) and modern (public_transport schema) stops.
• Confirmed the change compiles and archives successfully via the Build Debug IPA CI workflow on this branch.
• Tested on device
• Tested with fresh pairing file

Screenshots

N/A — backend data-fetching change, no UI changes.

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