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>

xtask/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "xtask"
+version = "0.1.0"
+edition = "2024"
+
+# E5 (RUST.md): packages app/shellApp into a signed, installable APK without
+# Gradle driving the assembly (cargo ndk -> javac -> d8 -> aapt2 -> zipalign
+# -> apksigner). No dependencies beyond the standard library: every step
+# below is "run this SDK tool with these arguments and check its exit
+# status," which needs nothing a crate would help with, and every tool
+# invoked is one this project already requires (the NDK, the SDK
+# build-tools, the JDK, `cargo ndk`) -- see AGENTS.md's "new dependencies
+# need a reason."
+[[bin]]
+name = "xtask"
+path = "src/main.rs"