Clean up before restarting.

Release caption model resources explicitly and reduce unused preview memory after backgrounding.

In this release

Improved
Idle caption engines release their model sessions before shutdown. Replacement engines wait for cleanup, with a bounded timeout if the old worker cannot respond.
Improved
Backgrounding clears unused completed previews from the memory cache. Saved previews remain on the device and can be restored without re-rendering.
Diagnostics
Reports distinguish completed cleanup, cleanup errors, timeout fallback, and forced termination.
Validation
225 tests and real desktop browser model disposal and restart checks pass. Verification of the background-return failure on affected iPhones is still needed.