Primary fix: checkState() now detects when both old and new states map
to the HOME page (DOWNLOADING_ASSETS and READY) and skips re-navigation.
This prevents destroying and recreating the Home page, which would
close the already-closed DownloadDone channel.
Defense-in-depth: Home.downloadDone flag prevents close if a new Home
page somehow reaches the close path.
12 application tests, 4 page tests, 18 mixer tests (34 total).
Tests cover: prerequisite priority ordering, state blocking on
downloads, non-blocking channel read, double-close guard, download
failure path, and repeated computeState calls.
