fix: complete hybridGPU wiring + make state-derived options overridable

Two related fixes that together close the "minimal wiring" gap behind
`nomarchy.system.features.hybridGPU`.

1. Complete the NVIDIA driver stack inside hardware.nix's hybridGPU
   mkIf block.

   Before: `hybridGPU = true` enabled supergfxd and... that was it.
   supergfxd manages mode switching by black/unblacklisting the nvidia
   kernel module, but without the rest of the NVIDIA stack actually
   loaded the dGPU has no driver to drive. Hyprland/Wayland silently
   stayed on the iGPU regardless of mode.

   After: hybridGPU=true also wires
     services.xserver.videoDrivers = ["nvidia"]   (loads the driver
                                                   under Wayland too)
     hardware.graphics.{enable,enable32Bit}
     hardware.nvidia.modesetting.enable           (required for
                                                   Wayland)
     hardware.nvidia.powerManagement.enable
     hardware.nvidia.package = config.boot.kernelPackages
                                      .nvidiaPackages.stable
     boot.kernelParams += "nvidia-drm.modeset=1"

   All wired with lib.mkDefault so a downstream system.nix can pin a
   beta driver, flip to the open kernel module, or set
   `hardware.nvidia.prime.{offload.enable, intelBusId, nvidiaBusId}`
   for render-offload. The bus IDs are per-machine (find via
   `lspci -D`) so they stay user-supplied; docs/OPTIONS.md has the
   full recipe.

2. Add lib.mkDefault to every state.json-derived assignment in
   core/system/state.nix and core/home/state.nix.

   Same priority bug on both sides: assignments like
   `features.hybridGPU = systemState.features.hybridGPU or false`
   landed at default priority. A downstream system.nix saying
   `nomarchy.system.features.hybridGPU = true` would then conflict
   with the state-derived value at the same priority, and Nix would
   refuse the merge with "conflicting definition values" — the
   user's override couldn't take effect.

   Verified by an explicit eval: extending the default nixosConfig
   with `nomarchy.system.features.hybridGPU = true` now resolves
   cleanly and the full driver stack engages.

   Side-effect: core/system/state.nix now reads from
   lib/state-schema.nix like the home side does, completing the
   schema-centralization started two batches ago.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docs/OPTIONS.md

@@ -53,7 +53,25 @@ Wired in `features/desktop/waybar/default.nix` (filters the battery widget out o
 
 ### `nomarchy.system.features.hybridGPU`
 
-`bool`, default `false`. Enables `services.supergfxd.enable` for laptops with switchable GPUs.
+`bool`, default `false`. NVIDIA-hybrid laptop support. Wires:
+
+- `services.supergfxd.enable` for runtime mode switching (`Integrated` / `Hybrid` / `Vfio` / `AsusEgpu`), driven by `nomarchy-toggle-hybrid-gpu`.
+- The NVIDIA driver stack (`services.xserver.videoDrivers = ["nvidia"]`, `hardware.graphics.{enable,enable32Bit}`, `hardware.nvidia.{modesetting,powerManagement}.enable`, `boot.kernelParams = ["nvidia-drm.modeset=1"]`).
+
+All driver knobs use `lib.mkDefault`, so a downstream `system.nix` can pin a beta driver or flip to the open kernel module without forking the module.
+
+**You still have to add bus IDs** — they're per-machine and can't be derived from any flag. Find them with `lspci -D | grep -E 'VGA|3D'`, then in your `/etc/nixos/system.nix`:
+
+```nix
+hardware.nvidia.prime = {
+  offload.enable = true;
+  offload.enableOffloadCmd = true;
+  intelBusId  = "PCI:0:2:0";   # or `amdgpuBusId` for AMD iGPU
+  nvidiaBusId = "PCI:1:0:0";
+};
+```
+
+Without prime config, supergfxd still switches modes but render-offload via `nvidia-offload <cmd>` is unavailable.
 
 ### `nomarchy.system.snapper.enable`