refactor(services): power-profiles-daemon replaces auto-cpufreq

amd-pstate in active mode reads only the min/max and EPP registers and
ignores the requested performance level, so the governor auto-cpufreq
switches decides nothing. ppd writes those registers and the ACPI
platform_profile; on the testvm, where neither exists, it falls back to
the placeholder driver instead of polling sysfs a virtio guest lacks.

The NixOS module asserts against auto-cpufreq, so the two cannot run at
the same knob. Matches the moonarch tree this flake deploys from.

modules/services.nix

@@ -20,7 +20,11 @@
   services.upower.enable = true;
   services.gvfs.enable = true;
   services.fwupd.enable = true;
-  services.auto-cpufreq.enable = true;
+  # power-profiles-daemon over auto-cpufreq: on amd-pstate in active mode the
+  # platform decides the frequency from the min/max and EPP registers alone, so
+  # the governor is the wrong knob. ppd writes EPP and the ACPI platform_profile;
+  # where neither exists it falls back to a driver that touches no hardware.
+  services.power-profiles-daemon.enable = true;
 
   # --- Firewall, replaces the ufw calls in post-install.sh ---
   networking.firewall.enable = true;