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/demo/AndroidManifest.xml

@@ -0,0 +1,23 @@
+<?xml version="1.0" encoding="utf-8"?>
+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+    package="com.catacomb.spike"
+    android:versionCode="1"
+    android:versionName="0.1-spike">
+
+    <uses-sdk android:minSdkVersion="24" android:targetSdkVersion="34" />
+
+    <application
+        android:label="Catacomb Spike"
+        android:icon="@android:drawable/sym_def_app_icon"
+        android:extractNativeLibs="true"
+        android:allowBackup="false">
+        <activity
+            android:name=".MainActivity"
+            android:exported="true">
+            <intent-filter>
+                <action android:name="android.intent.action.MAIN" />
+                <category android:name="android.intent.category.LAUNCHER" />
+            </intent-filter>
+        </activity>
+    </application>
+</manifest>