Fix imported two-thumb settings and gesture dictionary state staying stale

[AsafMah]

Describe the bug
The v4.2.0 integration review confirmed retained restore and dictionary-cache defects. This includes the reported case where importing normal-keyboard settings into EXP did not update two-thumb behavior. Restore blocks the main thread, can finalize while its worker is still running, and does not explicitly refresh already-loaded SettingsValues. Fallback gesture indices and spelling-validity caches also survive mutations or accept stale background builds.

To Reproduce

• Initialize runtime settings, then import a same-version backup changing combining grace/manual spacing. Compare persisted preferences with the active cached settings without restarting the IME.
• Use a delayed document stream: restore must not block Main or finalize before the worker completes.
• Restore an archive containing an escaping path: validate before any target deletion and preserve files outside the selected restore directory.
• Build fallback gesture and spelling caches, then block/promote a word; observe stale results without changing fields.
• Complete two layout-index builds in reverse order after invalidation; an obsolete result can overwrite the newer index.

Expected behavior

• Backup/restore never waits synchronously on Main; completion, migration and listener restoration happen exactly once in the correct order.
• Successful import immediately refreshes runtime two-thumb settings, preserving subtype/theme/toolbar state.
• Archive destinations are validated before destructive operations; errors leave listeners usable and never report success.
• Blocking/promoting words invalidates fallback snapshots after actual mutation completion.
• Stale build success, failure and cleanup cannot overwrite a newer gesture index.
• Spelling-validity caches reflect block/remove/reload/promotion immediately after mutation.
  Each fix requires a failing-before production-path regression. No new decoder algorithm or recognition-quality claim is part of this issue.

Screenshots
N/A — deterministic state, archive and executor fixtures.

App version & Flavor
Retained defects reviewed at stabilized dev 07ef7536f and the v4.2.0 integration. Correction branch starts at b376f5a17. Settings apply to all flavors; fallback-index issues apply when the Java fallback is selected.

Device Info

• Model: reporter uses SM-S936B; new review reproductions use isolated JVM/Robolectric fixtures.
• OS version: not asserted for this review.

[AsafMah]

Integration update: #155 includes the existing asynchronous validated restore/runtime-settings refresh and dictionary mutation/publication corrections. The maintainer explicitly authorized removing the Java fallback engine and its index machinery, so the fallback-index-specific requirements here are superseded rather than a reason to restore that backend. The working dual-thumb composition path and stored user data remain. No new recognition algorithm is included.