feat(client/android): CI screenshot capture via Roborazzi

Play-listing/marketing screenshots of the Compose client rendered on the host JVM
by Roborazzi (Robolectric Native Graphics) — no emulator, GPU, KVM, host, or JNI
core. Five scenes render the REAL composables with embedded mock state under a
forced brand palette (Material You has no wallpaper to seed from on the JVM):
hosts grid, settings, TOFU + PIN dialogs, and the live stats HUD. Validated 5/5
locally.

- New JVM unit-test source set (app/src/test) + Roborazzi/Robolectric test deps;
  @Config(sdk=36) is mandatory (no android-all jar for compileSdk 37) and the
  animation clock is paused so a text-bearing scene reaches idle.
- kit: `-PskipRustBuild` skips the cargo-ndk native build so the JVM-only test job
  needs no Rust/NDK; normal APK/AAR builds are unchanged.
- Widen BrandDark / StatsOverlay to internal so the tests can use them.
- Standalone best-effort tag-gated workflow; PNGs upload as a 30-day artifact.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

clients/android/app/src/main/kotlin/io/unom/punktfunk/StreamScreen.kt

@@ -319,7 +319,7 @@ fun StreamScreen(handle: Long, micEnabled: Boolean, onDisconnect: () -> Unit) {
  * `[fps, mbps, latP50Ms, latP95Ms, latValid, skew, w, h, hz, dropped]`.
  */
 @Composable
-private fun StatsOverlay(s: DoubleArray, modifier: Modifier = Modifier) {
+internal fun StatsOverlay(s: DoubleArray, modifier: Modifier = Modifier) {
     if (s.size < 10) return
     val w = s[6].toInt()
     val h = s[7].toInt()

clients/android/app/src/main/kotlin/io/unom/punktfunk/Theme.kt

@@ -10,7 +10,9 @@ import androidx.compose.ui.platform.LocalContext
 
 // punktfunk brand violets (from the app icon: #6C5BF3 / #A79FF8 / #D2C9FB on a #16132A indigo).
 // Used as the fallback dark scheme on pre-Android-12 devices; on 12+ we defer to Material You.
-private val BrandDark = darkColorScheme(
+// `internal` (not private) so the CI screenshot tests can force the deterministic brand palette —
+// Material You dynamic colour has no wallpaper to seed from under the Robolectric JVM renderer.
+internal val BrandDark = darkColorScheme(
     primary = Color(0xFFA79FF8),
     onPrimary = Color(0xFF1B1442),
     primaryContainer = Color(0xFF4C3FB3),