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.

modules/desktop/pulseaudio.nix

@@ -20,13 +20,21 @@
       # no need to redefine it in your config for now)
       #media-session.enable = true;
 
+      extraConfig.pipewire."99-quantum" = {
+        "context.properties" = {
+          "default.clock.quantum"     = 1024;
+          "default.clock.min-quantum" = 1024;
+          "default.clock.max-quantum" = 8192;
+        };
+      };
+
       wireplumber.configPackages = [
         (pkgs.writeTextDir "share/wireplumber/wireplumber.conf.d/99-alsa-nova-3.conf" ''
           monitor.alsa.rules = [
             {
               matches = [
                 {
-                  node.name = "alsa_output.usb-SteelSeries_Arctis_Nova_3-00.analog-stereo"
+                  node.name = "alsa_output.usb-SteelSeries_Arctis_Nova_7-00.analog-stereo"
                 }
               ]
               actions = {
@@ -34,7 +42,7 @@
                   audio.format = "S24LE"
                   audio.rate = 96000
                   api.alsa.period-size = 1024
-                  api.alsa.period-num = 4
+                  api.alsa.period-num = 8
                   api.alsa.disable-batch = false
                 }
               }