perf(android): low-latency decode overhaul — vendor keys, async loop, system tuning

Close the latency gap on the Android client with per-SoC decoder tuning, an
event-driven decode loop, and full system integration.

- Decoder selection: rank MediaCodecList decoders in Kotlin (hardware/vendor
  preferred, software avoided, FEATURE_LowLatency probed) and create the chosen
  one by name. Per-SoC low-latency keys gated on the codec-name prefix: Qualcomm
  picture-order + low-latency, Exynos (also Google Tensor), Amlogic, HiSilicon;
  MediaTek vdec-lowlatency set unconditionally. operating-rate = MAX (Qualcomm)
  vs priority = 0 (else) are mutually exclusive. NVIDIA/Rockchip/Realtek have no
  vendor key — covered by ranking + the standard low-latency key.

- Async decode loop: AMediaCodec async-notify replaces the poll loop, presenting a
  decoded frame the instant it is ready instead of waiting out a poll interval.
  Behind USE_ASYNC_DECODE with the synchronous loop kept for A/B during bring-up.

- System integration: Wi-Fi FULL_LOW_LATENCY lock and HDMI ALLM
  (setPreferMinimalPostProcessing) for the stream's lifetime; game_mode_config.xml
  opting out of OEM downscaling / FPS overrides.

- Pipeline: boost the data-plane pump + audio thread priorities, AAudio usage=Game,
  DSCP marking on by default on Android, ADPF setPreferPowerEfficiency(false),
  and setFrameRateWithChangeStrategy(ALWAYS) to force the HDMI mode switch on TV.

- lowLatencyMode master toggle (default on) as the escape hatch; the stats HUD now
  shows the resolved decoder name.

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

crates/punktfunk-core/src/transport/qos.rs

@@ -66,12 +66,18 @@ impl MediaClass {
     }
 }
 
-/// Whether DSCP/QoS marking is enabled (`PUNKTFUNK_DSCP=1`). Off by default.
+/// Whether DSCP/QoS marking is enabled. Default **on for Android**, **off elsewhere**: on Wi-Fi
+/// (where most Android clients live) access points commonly map DSCP to WMM access categories, so
+/// tagging the video/audio sockets can win real airtime priority against other traffic on the link;
+/// on the wired paths the other clients use it's rarely honoured and some paths bleach or reject
+/// marked packets, so it stays opt-in there. `PUNKTFUNK_DSCP` overrides either way — `1`/`true`/`on`
+/// forces it on, `0`/`false`/`off` forces it off (e.g. to rule QoS out while debugging a flaky AP).
 pub(crate) fn dscp_enabled() -> bool {
-    matches!(
-        std::env::var("PUNKTFUNK_DSCP").as_deref(),
-        Ok("1") | Ok("true") | Ok("on")
-    )
+    match std::env::var("PUNKTFUNK_DSCP").as_deref() {
+        Ok("1") | Ok("true") | Ok("on") => true,
+        Ok("0") | Ok("false") | Ok("off") => false,
+        _ => cfg!(target_os = "android"),
+    }
 }
 
 /// Best-effort: tag `socket`'s outgoing packets for prioritized delivery of its media class. A no-op