add audio-rt module to fight xruns under cpu load

EasyEffects DSP plugins (rnnoise, deepfilternet, crystalizer) drop audio
when the system is under load. Layered workaround with per-optimization
toggles for bisecting impact:

- cgroup-based CPU partitioning: dedicated audio.slice with AllowedCPUs
  pinning, restricted app/session/background slices for everything else
- rlimits so PipeWire's module-rt sets SCHED_FIFO 88 directly instead of
  going through RTKit's priority-10 ceiling
- persistent watcher that re-applies performance governor on audio cores
  (gamemoded/PPD keep resetting it)
- ananicy rule pinning easyeffects to nice -12 for non-RT DSP threads
- znver4-tuned rebuilds of easyeffects and its DSP deps

Master switch + per-feature toggles via lumpiasty.audioRt.*; all enabled
by default on acer host. None of this fully eliminates dropouts on this
thermally constrained laptop but each layer is independently testable.

hosts/acer.nix

@@ -71,6 +71,7 @@ rec {
     amdCpu = true;
     noMitigations = false;
     enablePulseaudio = true;
+    audioRt.enable = true;
     sshd = true;
     users.user = true;
     # users.drugi = true;