feat(android): live stats HUD + low-latency decode tuning

Stats HUD (mirrors the Apple client): the decode thread accumulates FPS, receive
throughput, and capture->client latency (p50/p95, skew-corrected) in Rust
(clients/android/native/src/stats.rs); nativeVideoStats drains a snapshot ~1 Hz
over JNI as a DoubleArray. StreamScreen renders a Compose overlay
(W*H@Hz / fps / Mb/s / latency, + dropped-under-loss), toggled by a Settings
switch (persisted, default on) or a 3-finger tap.

Performance (decode.rs):
- ANativeWindow_setFrameRate(refresh_hz): align display vsync to the stream rate
  (no 60-in-120 judder); safe since minSdk 31 >= API 30.
- Raise the decode thread toward URGENT_DISPLAY (best-effort setpriority) so
  background work can't preempt it under load.
- Codec low-latency hints KEY_PRIORITY=0 (realtime) + KEY_OPERATING_RATE.

Verified host-side: cargo build/clippy/fmt --workspace (the ungated stats + JNI
accessor). The android-gated decode.rs (NDK) and the Kotlin build only in CI
(android.yml: gradle + cargo-ndk) -- APIs verified against crate sources.

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

clients/android/kit/src/main/kotlin/io/unom/punktfunk/kit/NativeBridge.kt

@@ -75,6 +75,14 @@ object NativeBridge {
     /** Stop + join the decode thread without closing the session. No-op on `0`. */
     external fun nativeStopVideo(handle: Long)
 
+    /**
+     * Drain ~1 s of live decode stats for the on-stream HUD, or `null` when no decode thread runs.
+     * Returns 10 doubles:
+     * `[fps, mbps, latP50Ms, latP95Ms, latValid, skewCorrected, width, height, refreshHz, framesDropped]`
+     * (the two flags are 1.0/0.0). Poll ~1 Hz; each call resets the measurement window.
+     */
+    external fun nativeVideoStats(handle: Long): DoubleArray?
+
     /**
      * Start host→client audio: Opus decode → jitter ring → AAudio (LowLatency), all in Rust. No-op
      * if already started. Best-effort — a failure leaves video streaming.