yarn: declarative lact config

hosts/yarn/lact.nix

@@ -0,0 +1,45 @@
+{
+  pkgs,
+  lib,
+  ...
+}:
+let
+  # Composite GPU id used by lactd: <pci_id>-<subsys_id>-<slot>.
+  # ASRock RX 7800 XT (Navi 32). Verify with `lact cli list-gpus` if the
+  # PCI slot ever changes (e.g. after adding/removing PCIe devices).
+  gpuId = "1002:747E-1849:5326-0000:0a:00.0";
+in
+{
+  # LACT (Linux AMDGPU Configuration Tool): https://github.com/ilya-zlobintsev/LACT
+  # Config schema reference: <lact src>/docs/CONFIG.md.
+  # /etc/lact/config.yaml is a read-only symlink to /nix/store, so GUI "Apply"
+  # fails — settings are Nix-only. lactd auto-restarts on config change via the
+  # upstream module's restartTriggers.
+  services.lact = {
+    enable = true;
+    settings = {
+      # Pin to current Config schema; bump in lockstep with lact upgrades to
+      # avoid a v(N)→v(N+1) migration save() failing against the RO symlink.
+      version = 5;
+      daemon = {
+        log_level = "info";
+        admin_group = "wheel";
+      };
+      apply_settings_timer = 5;
+      gpus.${gpuId} = {
+        # ASRock RX 7800 XT (Navi 32). Voltage offset tuned via sweep with
+        # clpeak under perf=high (sustained DPM7); visible artifacting on the
+        # display engine appeared at -300 mV, so this sits 75 mV below that
+        # cliff for a real-world safety margin (cool clpeak < hot gaming).
+        voltage_offset = -150; # mV; kernel range on this card: -450..0
+        max_core_clock = 2400; # MHz; default boost ~2520
+        power_cap = 200.0; # W; default 220, max 280
+        performance_level = "auto";
+      };
+    };
+  };
+
+  # Keep the existing pre-start sleep that lets amdgpu sysfs settle before
+  # lactd probes it. Purely additive — the upstream module sets no ExecStartPre.
+  systemd.services.lactd.serviceConfig.ExecStartPre = "${lib.getExe pkgs.bash} -c \"sleep 3s\"";
+}