Fix inconsistent bus stop loading: switch route queries from around() to bbox

[dizzafizza]

Summary

Bus stops still loaded inconsistently after the previous chunking fix (#4) — sometimes working, sometimes not, with no obvious pattern. Investigating against the live Overpass API found two compounding root causes.

Changes

Root cause 1 — query cost scales with point density, not route length. Overpass's around(radius, point-list) operator costs scale with points-in-list × candidate-elements-in-area. A single chunk at the previous chunkMaxPoints=160 reliably timed out. But even much smaller chunks (25-60 points) took anywhere from 3s to 23s purely depending on how road-dense the area was — the same chunk size that was fine in a quiet area could time out in a busy one. That's exactly the "sometimes works, sometimes doesn't" behavior: no fixed point-count chunk size is safe everywhere.

Root cause 2 — Overpass server timeouts are silent. When Overpass's own [timeout:N] elapses, it replies HTTP 200 with an empty elements array and a remark field describing the error — not an error status. The client only checked the HTTP status code, so a timed-out chunk was silently indistinguishable from "no stops near this stretch of road," and was never retried or logged.

Fixes:

• Replace the around() query with a bounding-box query per chunk. A bbox test is an O(1) rectangle check per candidate regardless of route shape or point density, unlike around(). Verified live: a chunk that took 16-23s (or timed out) via around() resolved via bbox in 2-8s for the same area — including a deliberately pathological zigzag route whose bbox came out much larger than the route itself.
• Chunk the route by real-world distance (~5km) instead of point count, since query cost no longer depends on point density — this also simplifies chunk building (no separate query-string downsampling step needed).
• Decode the remark field and treat any non-nil value as a failure, so a server-side timeout is retried and logged instead of silently accepted as empty data.
• Retry each chunk once with a short backoff before giving up, since the public Overpass instance is a shared, load-dependent resource where a single slow response shouldn't cost that stretch of road its data.

Testing

• Verified the exact query shape and decoder assumptions against the live Overpass API across multiple route shapes: a realistic ~5km stretch (2.6s, correct bus stops/speeds decoded), a dense-area chunk that previously took 16-23s via around() (now 2-8s via bbox), and a deliberately pathological zigzag route (bbox much larger than the route, still resolved in under 8s).
• Confirmed the exact server-side timeout/remark behavior by intentionally forcing an Overpass timeout and inspecting the response.
• 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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