E5: package app/shellApp without Gradle (cargo xtask apk)

New xtask/ crate (no deps) runs cargo ndk -> javac -> d8 -> aapt2 ->
zipalign -> apksigner directly, signed with the same key build-apk.sh
uses. Both pass conditions proved on the ai-app-2 emulator: the xtask
APK installs over the Gradle-built shellApp, and the notification
service starts and posts a real notification while backgrounded.

Adds one printRuntimeClasspathJars task to shellApp/build.gradle.kts
(and a matching signingConfig) -- the one disclosed Gradle call the
xtask still makes, to resolve the AndroidX/​:link dependency graph.
That call's Kotlin compilation of :link as a side effect also answers
E3's open kotlinc question, so no Java port of ServerStore was needed.
Wires a second Apk component into .dev-updater.ron beside the existing
one. Full writeup in RUST.md's E5 box.

Co-Authored-By: Claude Sonnet <noreply@anthropic.com>

.gitignore

@@ -33,3 +33,10 @@ sessions/
 # iris, the in-house UI library, is vendored at iris/ and built by cargo.
 iris/target/
 iris/android-app/target/
+
+# E5's packaging xtask (RUST.md). `build/` above already covers
+# xtask/build/outputs/apk (the published APK, see apk.rs's module doc).
+# The repo root has no Cargo workspace, so this is xtask's own
+# intermediate working files (target/xtask/apk/...), not a shared one.
+xtask/target/
+/target/