feat(android): installable demo APK exercising the Rust JNI core

Add android/demo — a minimal, dependency-free app (plain Android Views,
no Gradle/AGP/AndroidX/Compose) that calls the Phase-4 JNI core on-device
and displays the results. build-apk.sh drives the SDK build-tools directly
(aapt2 -> javac -> d8 -> zip -> zipalign -> apksigner), compiling against
android.jar alone so it needs no network and tolerates a very new system
JDK by routing the SDK tools through a JDK <= 21.

The app has three buttons -> platformFromUrl/platformDirName, classifyError,
and vttParse, each calling libcatacomb_core.so and showing the JSON.

Verified on an Android 14 (API 34) x86_64 emulator: installs, launches,
loads the .so, and returns correct JSON on-device
({"dir_name":"channels","display_name":"YouTube","icon":"..."} for a
YouTube URL). Output APK (out/, git-ignored): catacomb-spike-debug.apk,
v2+v3 signed, minSdk 24, native code for arm64-v8a + x86_64.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

android/.gitignore

@@ -5,6 +5,10 @@ rust/catacomb_core/target/
 # rather than commit binaries.
 app/src/main/jniLibs/
 
+# Manual APK build outputs (APK, signing artifacts, debug keystore) — build
+# products, never committed.
+demo/out/
+
 # Gradle / Android build outputs (Phase 1, not yet scaffolded here)
 .gradle/
 build/