Add T490 hardware configuration and docs

- modules/t490.nix: TLP power management, CPU throttling (throttled), S3 sleep,
  remote Nix builders (cluster), fwupd, fprintd, fprintd, Syncthing, Tailscale,
  Intel VA-API, TrackPoint sensitivity, PipeWire defaults
- flake.nix: add nixos-hardware input, import T490 module + nixos-hardware
- docs/T490-HARDWARE.md: WiFi variants, graphics, audio, fingerprint, sleep guide
- docs/T490-BATTERY.md: TLP config, threshold strategy, real-world battery numbers
- docs/T490-OFFLOADING.md: local vs cluster split, remote builder setup
- MIGRATION.md: add T490 post-install steps (fwupd, fprint, remote build key)
- README.md: reflect new module and docs structure

modules/t490.nix

@@ -0,0 +1,157 @@
+{ config, pkgs, lib, ... }:
+
+{
+  # === T490-specific hardware configuration ===
+  # Requires nixos-hardware flake input: nixos-hardware.nixosModules.lenovo-thinkpad-t490
+  # The nixos-hardware module provides:
+  #   - services.throttled (CPU throttling fix)
+  #   - PulseAudio profile set (5 simultaneous sinks)
+  #   - common/cpu/intel (VA-API drivers, microcode)
+
+  # === Kernel parameters ===
+  boot.kernelParams = [
+    "mem_sleep_default=deep"       # prefer S3 deep sleep over s2idle
+    "pcie_aspm=force"              # PCIe Active State Power Management
+    "i915.enable_fbc=1"            # frame buffer compression (power saving)
+    "i915.enable_psr=1"            # panel self-refresh (power saving)
+    "i915.enable_rc6=7"            # maximum GPU render power saving
+  ];
+
+  boot.kernelModules = [ "thinkpad_acpi" ];
+  boot.blacklistedKernelModules = [
+    "iTCO_wdt"
+    "iTCO_vendor_support"
+  ];
+
+  # === Display DPI ===
+  services.xserver.dpi = 157;     # 14" 1920×1080 IPS = 157 DPI
+
+  # === Firmware updates ===
+  services.fwupd.enable = true;
+
+  # === Fingerprint reader (Synaptics 06cb:00bd) ===
+  # Requires firmware update via fwupd first
+  services.fprintd.enable = true;
+
+  # === TLP — ThinkPad power management ===
+  # nixos-hardware thinkpad module provides base config; tune here
+  services.tlp = {
+    enable = true;
+    settings = {
+      CPU_SCALING_GOVERNOR_ON_AC = "performance";
+      CPU_SCALING_GOVERNOR_ON_BAT = "powersave";
+      CPU_ENERGY_PERF_POLICY_ON_AC = "balance_performance";
+      CPU_ENERGY_PERF_POLICY_ON_BAT = "power";
+      CPU_MAX_PERF_ON_AC = 100;
+      CPU_MAX_PERF_ON_BAT = 40;      # cap at ~1.7 GHz on battery
+      CPU_BOOST_ON_AC = 1;
+      CPU_BOOST_ON_BAT = 0;           # disable turbo on battery
+      START_CHARGE_THRESH_BAT0 = 40;  # battery health: start charging below 40%
+      STOP_CHARGE_THRESH_BAT0 = 80;   # battery health: stop at 80%
+      WIFI_PWR_ON_AC = "off";
+      WIFI_PWR_ON_BAT = "on";
+      RUNTIME_PM_ON_AC = "on";
+      RUNTIME_PM_ON_BAT = "auto";
+      USB_AUTOSUSPEND = 1;
+      INTEL_GPU_MIN_FREQ_ON_AC = 300;
+      INTEL_GPU_MAX_FREQ_ON_AC = 1100;
+      INTEL_GPU_MIN_FREQ_ON_BAT = 300;
+      INTEL_GPU_MAX_FREQ_ON_BAT = 600;
+    };
+  };
+
+  # === powertop ===
+  powerManagement.powertop.enable = true;
+
+  # Skip thermald — conflicts with throttled (enabled by nixos-hardware)
+  services.thermald.enable = false;
+
+  # === zram swap (faster + saves NVMe writes) ===
+  zramSwap = {
+    enable = true;
+    memoryPercent = 50;
+  };
+
+  # === Intel GPU — hardware video acceleration ===
+  hardware.graphics = {
+    enable = true;
+    extraPackages = with pkgs; [
+      intel-media-driver    # modern VA-API (iHD)
+      intel-vaapi-driver    # legacy VA-API (i965)
+      libvdpau-va-gl        # VDPAU → VA-API bridge
+    ];
+  };
+
+  environment.sessionVariables = {
+    LIBVA_DRIVER_NAME = "iHD";
+    MOZ_ENABLE_WAYLAND = "1";
+  };
+
+  # === Suspend/resume: reload WiFi after wake ===
+  powerManagement.powerUpCommands = ''
+    ${pkgs.kmod}/bin/modprobe -r iwlmvm iwlwifi || true
+    ${pkgs.kmod}/bin/modprobe iwlwifi || true
+  '';
+
+  # === Remote Nix builders (k3s cluster) ===
+  nix.distributedBuilds = true;
+  nix.buildMachines = [
+    {
+      hostName = "n97-mini";
+      system = "x86_64-linux";
+      maxJobs = 4;
+      supportedFeatures = [
+        "nixos-test"
+        "benchmark"
+        "big-parallel"
+        "kvm"
+      ];
+      sshUser = "root";
+      sshKey = "/root/.ssh/remote-build";
+    }
+    {
+      hostName = "n97";
+      system = "x86_64-linux";
+      maxJobs = 4;
+      supportedFeatures = [
+        "nixos-test"
+        "benchmark"
+        "big-parallel"
+        "kvm"
+      ];
+      sshUser = "root";
+      sshKey = "/root/.ssh/remote-build";
+    }
+  ];
+
+  # === Syncthing (Obsidian vault sync) ===
+  services.syncthing = {
+    enable = true;
+    user = "sscoble";
+    dataDir = "/home/sscoble/.syncthing";
+    configDir = "/home/sscoble/.syncthing/config";
+  };
+
+  # === Tailscale (consistent cluster access from any network) ===
+  services.tailscale.enable = true;
+
+  # === NVMe tools ===
+  environment.systemPackages = with pkgs; [ nvme-cli ];
+
+  # === Touchpad & TrackPoint ===
+  services.xserver.libinput = {
+    enable = true;
+    touchpad = {
+      tapping = true;
+      naturalScrolling = true;
+      accelSpeed = "0.3";
+      disableWhileTyping = true;
+    };
+  };
+
+  # TrackPoint sensitivity
+  services.udev.extraRules = ''
+    ACTION=="add", SUBSYSTEM=="input", ATTR{name}=="*TrackPoint*", \
+      ATTR{device/sensitivity}="200", ATTR{device/speed}="150"
+  '';
+}