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.

home-modules/pc.nix

@@ -41,6 +41,11 @@
     ];
     programs.librewolf.enable = true;
     services.easyeffects.enable = true;
+    systemd.user.services.easyeffects.Service = lib.mkIf osConfig.lumpiasty.audioRt.cpuPartitioning {
+      # Move easyeffects into audio.slice (defined in modules/desktop/audio-rt.nix)
+      # which has AllowedCPUs=<audioCpus> — pins all DSP work to the reserved cores.
+      Slice = "audio.slice";
+    };
 
     programs.chromium.enable = true;
     programs.chromium.package = pkgs.ungoogled-chromium;