Coredump from a reboot during a route replay: the replay re-centred the map onto
an un-cached area, the fetch task wrote new z16 tiles, and FS::mkdir faulted
inside lfs_alloc — a full/fragmented LittleFS faults rather than returning NOSPC.
The tile cache had no free-space guard or eviction, so heavy map use fills the
4.75 MB partition until the next mkdir reboots the device.
Add tilesFsLowSpace() (cached ~5s, fetch-task-only) and skip tile writes when the
cache has < 320 KB free. Reads keep working so cached areas still display; only
new tiles stop caching once full. The crashing mkdir is never reached.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
