terrainStorageDirectory
App-internal storage, so downloaded terrain is private to the app and removed with it.
Where this platform's offline terrain (Mapterhorn tiles plus OfflineTerrainRegion's manifest) is kept.
Mirrors org.meshtastic.feature.map.layers.mapLayersDirectory's expect/actual shape — same reasoning: app-private storage on Android, a home-directory folder on Desktop next to the existing ~/.meshtastic data. Declared here rather than in :feature:map-terrain (which owns org.meshtastic.feature.map.terrain.TerrainTileStore itself but deliberately knows nothing about where a caller puts it) or :feature:map (shared by both flavors, and the Google flavor's own offline-terrain storage convention is a separate piece of work, not this one's to decide).
org.meshtastic.feature.map.layers.mapLayerFileSystem is reused for the okio.FileSystem — it is already FileSystem.SYSTEM on every platform this module targets, and redeclaring the same expect/actual here would just be a second name for it.
Alongside the desktop database and imported map layers, which already live in ~/.meshtastic.