Reuse shared Polyline cache for focused shapes; drop decoded double-cache (#1908)

[bmander]

Addresses item 2 of #1908 (medium-severity debt review finding 8) — the first of three separate PRs closing that issue.

Problem

A focused trip's shape was cached twice under the same shapeId, with divergent policies:

• TripObservationRepository's ShapeCache — Polyline, LRU 100, no TTL (GTFS shapes are immutable; rationale documented in ShapeCache.kt).
• FocusedTripRepository.shapeCache — decoded List<GeoPoint>, LRU 32, with an unexplained 10-minute TTL on effectively-immutable data.

ensureShape() already returns the shared, deduped Polyline (coalescing the network fetch via its own SingleFlight), so the decoded cache was a redundant second copy: ~2× memory per focused shape, a second eviction policy, and a freshness threshold that contradicts the immutability rationale one layer down.

Change

• Drop FocusedTripRepository.shapeCache; fetchShape decodes on demand from the shared Polyline cache. The Location → GeoPoint map is cheap and runs on a focus change, not the per-frame render path.
• Keep the fetch-concurrency throttle (shapePermits) and same-shape decode coalescing (shapeFetches) — those are concurrency controls, not caching, so behavior beyond the removed cache is unchanged.
• routeStopCache and its ExpiringLruCache/TTL are untouched here (the stops-for-route cache is item 1 of Focused-trip data layer: stops-for-route fetched via two paths, shapes double-cached, focusedTrips in three copies #1908, a separate PR).

Tests

The cross-call shape memo now lives entirely in TripObservationRepository (already covered by TripObservationRepositoryTest). The FocusedTripRepositoryTest case that asserted FocusedTripRepository's own memo is reframed to document the delegation. The within-getGeometry shared-shape dedup test is unchanged and still passes.

• ./gradlew :onebusaway-android:compileObaGoogleDebugKotlin -PwarningsAsErrors=true — clean
• ./gradlew :onebusaway-android:testObaGoogleDebugUnitTest --tests '*FocusedTripRepositoryTest' --tests '*TripObservationRepositoryTest' --tests '*ShapeCacheTest' — green

Follow-ups (same issue, separate PRs)

• Item 1: one caching stops-for-route repository both the map and route-info paths project from.
• Item 3: derive the focused-trip set from the focus state flow (remove the scattered manual resets).

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