Fix the workspace build off-device

Two breaks that only appeared on a full `cargo test --workspace`.

`slint::android` exists only when compiling for Android, so darkroom-android
failed to compile on the host even though it is a workspace member. The entry
point is now gated on the target rather than on a feature.

The timeline forwarded `scrub` where the Timeline component declares
`scrub-to`, which the Slint compiler rejects.

Assisted-by: LLM

apps/darkroom-android/android/AndroidManifest.xml

@@ -0,0 +1,47 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+  DarkRoom Android manifest.
+
+  Deliberately minimal: this packages the viewer for on-device testing (spike
+  S2 needs Adreno and Mali hardware, which no emulator represents). Nothing
+  here is a distribution manifest yet — no permissions are declared because
+  the library grid reads through SAF, which grants per-tree rather than by
+  manifest permission (ARCH §6.9).
+-->
+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+    package="paris.tourolle.darkroom">
+
+    <!-- Vulkan 1.1 is what wgpu needs; the API 28 floor is where support is
+         dependable (NFR-COMPAT-1). Marked required so an unsupported device
+         fails at install rather than at first frame. -->
+    <uses-feature
+        android:name="android.hardware.vulkan.version"
+        android:version="0x00401000"
+        android:required="true" />
+
+    <application
+        android:label="DarkRoom"
+        android:hasCode="true"
+        android:allowBackup="false"
+        android:supportsRtl="true">
+
+        <!-- NativeActivity rather than a Kotlin Activity: android-activity's
+             glue loads libdarkroom.so and calls android_main. `android.app.lib_name`
+             is how it learns which library to load, and must match [lib].name. -->
+        <activity
+            android:name="android.app.NativeActivity"
+            android:exported="true"
+            android:configChanges="orientation|keyboardHidden|screenSize|screenLayout|density|uiMode"
+            android:windowSoftInputMode="adjustResize">
+
+            <meta-data
+                android:name="android.app.lib_name"
+                android:value="darkroom" />
+
+            <intent-filter>
+                <action android:name="android.intent.action.MAIN" />
+                <category android:name="android.intent.category.LAUNCHER" />
+            </intent-filter>
+        </activity>
+    </application>
+</manifest>

apps/darkroom-android/src/lib.rs

@@ -9,6 +9,11 @@
 //! * Logging goes to logcat. `env_logger` writes to stderr, which Android
 //!   discards.
 
+// `slint::android` exists only when compiling for Android, so the whole entry
+// point is gated on the target rather than on a feature. Without this the
+// crate is still a workspace member on the host, and `cargo test --workspace`
+// fails to compile it — a build break that only ever appears off-device.
+#[cfg(target_os = "android")]
 /// TRACES: M-13 | M-14
 /// Android application entry point, called by android-activity's glue.
 #[no_mangle]