Fixes a real race in FileTileCache where two overlapping put() calls
(the persistent background map and a freshly-opened Route Planner map
both fetching tiles at once) could interleave at the _saveManifest
await point and silently lose a tile from the on-disk manifest. All
mutating and reading operations now go through a single serialization
queue. Also logs the URL and cause of tile fetch failures in
CachedTileProvider before rethrowing, and adds a concurrency
regression test. On-device verification was not performed (no
adb/emulator access in this environment); see the ticket's Outcome
section.
