fix(ci): limit debug APK to arm64-v8a so apksigner has disk to write

The previous run got all the way through compile + link + zipalign and
then died inside apksigner with `IOException: No space left on device`.
Cross-compiling all three Android ABIs (arm64-v8a, armeabi-v7a, x86_64)
in debug mode blows target/ past 25 GB, and by the time apksigner is
streaming the signed APK to disk the runner has nothing left.

Two changes:

  1. build_android_apk.sh now reads `ABIS` from the environment (defaults
     to all three for backwards compat) and uses it to assemble the
     cargo-ndk `-t` flags.
  2. android-build.yml passes ABIS=arm64-v8a, since the debug artifact
     is consumed by adb-installing to a single arm64 device and the
     other two ABIs were dead weight.

Also frees \$STAGING/app-unsigned.apk right after zipalign so it's not
sitting next to the aligned APK and the output APK during signing.

Release workflow is untouched — release APKs still ship all three ABIs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/android-build.yml

@@ -104,6 +104,10 @@ jobs:
         run: cargo install cargo-ndk --locked
 
       # ── Build APK ──────────────────────────────────────────────────────
+      # Debug CI only builds arm64-v8a — full three-ABI debug builds blow
+      # past the runner's disk budget (~25 GB of target/ + intermediate
+      # APKs caused apksigner to OOM-on-disk in the previous run). Release
+      # CI still ships all three ABIs from android-release.yml.
       - name: Build debug APK
         env:
           ANDROID_HOME: ${{ env.ANDROID_SDK }}
@@ -111,6 +115,7 @@ jobs:
           BUILD_TOOLS_VERSION: ${{ env.BUILD_TOOLS_VERSION }}
           PLATFORM: ${{ env.PLATFORM }}
           PROFILE: debug
+          ABIS: arm64-v8a
         run: ./scripts/build_android_apk.sh
 
       # ── Artifact ───────────────────────────────────────────────────────