P5.8: add an Android .apk bundle smoke test to the corpus (first Android bundler coverage)

Mirror the macOS .app smoke test (1665) for Android. New `.build` `apk`
directive (ApkCheck = { out, bundle_id, expect }) cross-compiles via
`sx build --target android --apk ... --bundle-id ... -o lib*.so`, then
asserts the produced APK's zip entries (AndroidManifest.xml, classes.dex,
lib/arm64-v8a/) via `unzip -l`. Build+inspect only — aarch64-linux-android
can't execute on the host, so no exit/stdout/stderr snapshot; the apk
branch is self-contained and never falls through to stream comparison.

Gated on the Android SDK ($ANDROID_HOME / $ANDROID_SDK_ROOT /
~/Library/Android/sdk) AND a real JDK (`javac -version` exit 0 — the
macOS /usr/bin/javac stub fails the gate). Missing either → skip cleanly,
so a bare-host `zig build test` stays green. Cleanup rm -rf's the apk,
staged .so, .stage dir, .unaligned/.aligned intermediates, and the
apksigner .idsig sidecar.

Verified: default `zig build test` skips 1666 (709 examples ran, 0 failed;
476/476 unit). With JAVA_HOME set to Android Studio's jbr, 1666 RUNS and
PASSES (apk built + all three entries found).

examples/expected/1666-platform-android-apk-smoke.build

@@ -0,0 +1 @@
+{ "apk": { "out": ".sx-tmp/1666-platform-android-apk-smoke.apk", "bundle_id": "co.swipelab.sxapksmoke", "expect": ["AndroidManifest.xml", "classes.dex", "lib/arm64-v8a/"] } }