ci(android): install libdbus, cache cargo and the staged libmpv

Every Android run since August failed in :app:buildRustHost: the host
build of pstr-android reaches libdbus-sys through keyring's Secret
Service store, and the runner had no libdbus-1-dev. With no successful
run, nothing was ever cached, so each run rebuilt from nothing.

- libdbus-1-dev in both jobs
- one shared, save-on-failure Rust cache for both jobs
- the staged libmpv tree cached on the build script's hash; the script
  stamps its output with the revision and its own hash and skips the
  libmpv build when the stamp matches

PLAN.md records the Android parity and UI work that follows.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/build-libmpv-android.sh

@@ -7,6 +7,20 @@ source_dir="${PSTR_MPV_SOURCE_DIR:-$repo_root/.build/mpv-android}"
 revision=20a3fa526fac6d3fe267aee0d4c349893fee65a3
 destination="$repo_root/android/app/build/generated/mpv"
 
+# The staged tree depends only on the pinned revision and on this script, which
+# also pins the NDK. A tree stamped with both is reused as is: that is what lets
+# CI restore it from a cache instead of rebuilding FFmpeg and every other
+# dependency from source on each run.
+stamp="$revision $(sha256sum "${BASH_SOURCE[0]}" | cut -d' ' -f1)"
+if [[ "$(cat "$destination/STAMP" 2>/dev/null)" == "$stamp" &&
+      -f "$destination/jniLibs/arm64-v8a/libmpv.so" &&
+      -f "$destination/jniLibs/x86_64/libmpv.so" &&
+      -f "$destination/mpv-android-source-$revision.tar.gz" ]]; then
+  echo "libmpv $revision is already staged in $destination"
+  exit 0
+fi
+rm -f -- "$destination/STAMP"
+
 if [[ ! -d "$source_dir/.git" ]]; then
   git clone https://github.com/mpv-android/mpv-android.git "$source_dir"
 fi
@@ -191,3 +205,5 @@ tar \
   -czf "$destination/mpv-android-source-$revision.tar.gz" \
   .
 printf '%s\n' "$revision" > "$destination/REVISION"
+# Last, so an interrupted run never leaves a stamp over a partial tree.
+printf '%s\n' "$stamp" > "$destination/STAMP"