E3: the Kotlin/Java shell over a JNI bridge into Rust (RUST.md)

Two Java classes (MainActivity, NotificationService) hand their lifecycle
to a new android-shell crate built on client-core; client-core gains
notifications.rs (the /notifications SSE parse and attention_line, ported
from Notifications.kt). Packaged as a new app/shellApp Gradle module
rather than a rewrite of app/androidApp in place, so that module's working
Compose UI is untouched.

Both pass conditions held on the emulator: a notification arrived in
Android's drawer with the app closed, and a shared text share landed as a
real message in a sandbox session's transcript. Found and fixed three
real bugs along the way (a silently-wrong JNI signature from a generic
JObject parameter, a class-by-name lookup failing on this crate's own
background thread for lack of an app ClassLoader, and onStartCommand
opening two /notifications connections per enrollment -- the last a
latent bug in Notifications.kt itself). Full account, exact commands and
what was deliberately cut are in RUST.md's E3 box.

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

app/shellApp/build.gradle.kts

@@ -0,0 +1,115 @@
+plugins { alias(libs.plugins.androidApplication) }
+
+// E3 (RUST.md): the Kotlin/Java shell being replaced by a thin JNI bridge
+// into Rust (`../../android-shell`). Deliberately its own module rather
+// than a rewrite of `:androidApp` in place -- that module is ~13,000 lines
+// of working Compose UI this experiment does not touch, and the two can be
+// installed side by side on the same development device (see
+// `settings.SCHEME`'s doc in `android-shell` for why the deep-link scheme
+// and Keystore alias are not the production app's). No Compose plugin, no
+// Kotlin source of its own: `MainActivity`/`NotificationService` are plain
+// Java, and the CA constant below is generated as Java too.
+//
+// The CA this build pins is baked in the same way `androidApp`'s does --
+// see that module's `build.gradle.kts` comment for the reasoning (the
+// trust boundary follows the machine that builds, never a pasted copy).
+// `PinnedCa.java`'s package must match `android-shell`'s
+// `settings::load_pinned_ca` lookup (`com/example/aiapp/shell/PinnedCa`).
+val pinnedCaPath: String =
+    System.getenv("AI_APP_CA")
+        ?: "${System.getenv("XDG_CONFIG_HOME") ?: "${System.getProperty("user.home")}/.config"}" +
+            "/ai-app/certs/ca.pem"
+
+abstract class GeneratePinnedCa : DefaultTask() {
+    @get:Input abstract val caPath: Property<String>
+
+    @get:InputFile
+    @get:Optional
+    @get:PathSensitive(PathSensitivity.NONE)
+    abstract val caCertificate: RegularFileProperty
+
+    @get:OutputDirectory abstract val outputDir: DirectoryProperty
+
+    @TaskAction
+    fun generate() {
+        val path = caPath.get()
+        val ca = File(path)
+        if (!ca.isFile) {
+            throw GradleException(
+                "No CA certificate at $path.\n" +
+                    "Start ai-server (or app/ui-sandbox.sh) once on this machine first -- it " +
+                    "generates the CA this build pins.\n" +
+                    "Set AI_APP_CA=/path/to/ca.pem to build against a different one."
+            )
+        }
+        val pem = ca.readText().trim()
+        if (!pem.startsWith("-----BEGIN CERTIFICATE-----")) {
+            throw GradleException("$path is not a PEM certificate.")
+        }
+        val dir = outputDir.get().dir("com/example/aiapp/shell").asFile
+        dir.mkdirs()
+        // Same reasoning as androidApp's generatePinnedCert: the text block
+        // must start immediately after the opening `"""`, or
+        // CertificateFactory stops recognising the "-----BEGIN" preamble.
+        File(dir, "PinnedCa.java")
+            .writeText(
+                """
+                |// Generated from $path by the generatePinnedCa task. Do not edit.
+                |package com.example.aiapp.shell;
+                |
+                |public final class PinnedCa {
+                |    private PinnedCa() {}
+                |    public static final String PINNED_CA_PEM = ""${'"'}
+                |$pem""${'"'};
+                |}
+                |"""
+                    .trimMargin()
+            )
+    }
+}
+
+val generatePinnedCa =
+    tasks.register<GeneratePinnedCa>("generatePinnedCa") {
+        val ca = file(pinnedCaPath)
+        caPath.set(pinnedCaPath)
+        if (ca.isFile) {
+            caCertificate.set(ca)
+        }
+    }
+
+android {
+    namespace = "com.example.aiapp.shell"
+    compileSdk = 37
+
+    defaultConfig {
+        applicationId = "com.example.aiapp.shell"
+        minSdk = 24
+        targetSdk = 37
+        versionCode = 1
+        versionName = "1.0"
+    }
+    compileOptions {
+        sourceCompatibility = JavaVersion.VERSION_21
+        targetCompatibility = JavaVersion.VERSION_21
+    }
+}
+
+androidComponents {
+    onVariants { variant ->
+        variant.sources.java?.addGeneratedSourceDirectory(generatePinnedCa, GeneratePinnedCa::outputDir)
+    }
+}
+
+dependencies {
+    // The Keystore-sealed enrollment (ServerStore/ServerSettings) --
+    // android-shell's settings.rs calls into this Kotlin class directly
+    // over JNI rather than re-sealing the token in Rust; see that file's
+    // module doc.
+    implementation(project(":link"))
+    // NotificationCompat/NotificationManagerCompat/NotificationChannelCompat/
+    // ServiceCompat -- android-shell's notify.rs calls these classes over
+    // JNI so the pre-26 fallback behaviour (no channels) lives once, in
+    // the library that already has it, rather than being re-derived as a
+    // set of Build.VERSION.SDK_INT branches in Rust.
+    implementation(libs.androidx.core.ktx)
+}