ci(android): keep the cached libmpv and Rust target trees

Gradle deletes the declared outputs of a task it has no history for, so
on every fresh checkout buildLibmpvAndroid threw away the libmpv tree just
restored from the cache and rebuilt it from source (~14 min per job). The
task no longer declares outputs; the script's STAMP decides.

rust-cache treats target/android/<triple> as a profile directory and
deletes its release tree before saving, so both cargo-ndk ABIs were
rebuilt from scratch on every run (~6 min per job). It now caches only the
registry and helpers; the target directory goes through actions/cache,
keyed on toolchain and lockfile and restoring from the last lockfile.

.github/workflows/android.yml

@@ -107,19 +107,34 @@ jobs:
           "build-tools;${ANDROID_BUILD_TOOLS}"
           "ndk;${ANDROID_NDK}"
 
-      - uses: dtolnay/rust-toolchain@stable
+      - id: toolchain
+        uses: dtolnay/rust-toolchain@stable
         with:
           toolchain: '1.96'
           targets: aarch64-linux-android,x86_64-linux-android
 
-      # One cache for both jobs: the signed build compiles the same crates for
-      # the same targets. Saved on failure too, so a run that breaks late does
-      # not throw away everything it compiled before the break.
+      # The registry and the installed helpers only. rust-cache reads
+      # `target/android/<triple>` as a profile directory and deletes the
+      # `release` tree under it before saving, so with its target caching on
+      # every run rebuilt both ABIs from scratch.
       - uses: Swatinem/rust-cache@v2.9.1
         with:
-          shared-key: android
+          shared-key: android-registry
+          cache-targets: false
           cache-on-failure: true
 
+      # The target directory as is: the host build the UniFFI bindings come
+      # from and both cargo-ndk ABIs. One entry for both jobs, since the signed
+      # build compiles the same crates for the same targets. A new lockfile
+      # starts from the last one's tree, so a version bump rebuilds only the
+      # workspace crates.
+      - id: target
+        uses: actions/cache/restore@v6
+        with:
+          path: target
+          key: rust-target-android-${{ steps.toolchain.outputs.cachekey }}-${{ hashFiles('Cargo.lock') }}
+          restore-keys: rust-target-android-${{ steps.toolchain.outputs.cachekey }}-
+
       # libmpv and its dependency tree are the slowest part of the build, and
       # depend only on the pinned revision and the build script, which stamps
       # its output with both and skips the build when the stamp matches. Built
@@ -167,6 +182,12 @@ jobs:
       - name: Android checks and artifacts
         run: bash scripts/build-android.sh all
 
+      - if: steps.target.outputs.cache-hit != 'true'
+        uses: actions/cache/save@v6
+        with:
+          path: target
+          key: ${{ steps.target.outputs.cache-primary-key }}
+
   signed-release:
     name: Signed release
     if: startsWith(github.ref, 'refs/tags/v') || inputs.signed_release
@@ -209,19 +230,34 @@ jobs:
           "build-tools;${ANDROID_BUILD_TOOLS}"
           "ndk;${ANDROID_NDK}"
 
-      - uses: dtolnay/rust-toolchain@stable
+      - id: toolchain
+        uses: dtolnay/rust-toolchain@stable
         with:
           toolchain: '1.96'
           targets: aarch64-linux-android,x86_64-linux-android
 
-      # One cache for both jobs: the signed build compiles the same crates for
-      # the same targets. Saved on failure too, so a run that breaks late does
-      # not throw away everything it compiled before the break.
+      # The registry and the installed helpers only. rust-cache reads
+      # `target/android/<triple>` as a profile directory and deletes the
+      # `release` tree under it before saving, so with its target caching on
+      # every run rebuilt both ABIs from scratch.
       - uses: Swatinem/rust-cache@v2.9.1
         with:
-          shared-key: android
+          shared-key: android-registry
+          cache-targets: false
           cache-on-failure: true
 
+      # The target directory as is: the host build the UniFFI bindings come
+      # from and both cargo-ndk ABIs. One entry for both jobs, since the signed
+      # build compiles the same crates for the same targets. A new lockfile
+      # starts from the last one's tree, so a version bump rebuilds only the
+      # workspace crates.
+      - id: target
+        uses: actions/cache/restore@v6
+        with:
+          path: target
+          key: rust-target-android-${{ steps.toolchain.outputs.cachekey }}-${{ hashFiles('Cargo.lock') }}
+          restore-keys: rust-target-android-${{ steps.toolchain.outputs.cachekey }}-
+
       # libmpv and its dependency tree are the slowest part of the build, and
       # depend only on the pinned revision and the build script, which stamps
       # its output with both and skips the build when the stamp matches. Built
@@ -266,6 +302,12 @@ jobs:
       - name: Build signed APK and AAB
         run: bash scripts/build-android.sh signed
 
+      - if: steps.target.outputs.cache-hit != 'true'
+        uses: actions/cache/save@v6
+        with:
+          path: target
+          key: ${{ steps.target.outputs.cache-primary-key }}
+
       # The APK bundles a GPL libmpv, so its corresponding source ships next to
       # it. The AAB is for Play only and stays out of the GitHub release.
       - name: Stage release files