fix(settings): stop wiping user settings on data re-extraction (DasherCore v0.2.10)

[willwade]

Summary

Fixes the Android side of the user report: "the input filter doesn't remember your choice — every time I start Dasher I have to switch to stylus mode."

Root cause

The bundled data directory was passed as both data_dir and user_dir to the engine, and DataInstaller deleteRecursively()s that directory on every DATA_VERSION bump — which accompanies every DasherCore pin bump. So every app update destroyed dasher_settings.xml: input filter, speed, alphabet, everything. This violates DasherCore's CAPI contract, which explicitly keeps the read-only data directory and the user-writable directory (settings, training deltas) distinct.

Changes

• DataInstaller.userDir(context) — dedicated filesDir/dasher_user that re-extraction can never touch. Both engine creation sites (MainActivity, DasherImeService) now pass it as the user dir.
• One-time migration — migrateUserArtifacts copies dasher_settings.xml and root-level user training_*.txt files out of the old shared dir before any wipe. Context-free (pure file operations) so it's unit-testable without Robolectric.
• Tests — DataInstallerMigrationTest: migration moves settings + user training but not bundled training-subdir files; no-op cases for missing/empty state.
• DasherCore → v0.2.10 — picks up the paragraph-newline alphabet fix (fix(alphabets): paragraph symbol outputs newline, not the pilcrow DasherCore#66), the other half of the same user-report batch ("return just shows a paragraph symbol").

Verification

• :app:testDebugUnitTest green (3 new migration tests)
• :app:assembleDebug green

Known follow-up (not fixed here)

The Activity and IME service each hold an engine instance against the same settings file with whole-file immediate saves — whichever saves last wins with its possibly-stale values. A stale-instance clobber window remains when both are alive simultaneously. Needs either a dasher_reload_settings CAPI or single-engine routing; tracked separately.

Type of change

• Bug fix

Definition of Done

• Build + unit tests green
• Commits signed off (DCO)