game-mods: drop in-house launchOptions writer, hardcode FH5 ini

Replaces three handfuls of custom code with upstream / static data:

- Per-app Steam launch options now declared via different-name/steam-
  config-nix's `programs.steam.config.apps.<n>` instead of a custom
  ~70-line `apply_launch_options` Python function. The dropped writer
  was racy: it edited localconfig.vdf without checking for a running
  Steam, so any timer firing while Steam was open would lose its
  changes on the next Steam shutdown. steam-config-nix's `closeSteam`
  flag closes that race.

  Also moves the GE-Proton compat-tool pin to declarative config —
  one fewer manual click in Steam UI to remember.

- `mods.<>.launchOptions` option, the `launchOptionsData` aggregation,
  and `LAUNCH_OPTIONS_DATA` are removed from desktop-game-mods.nix.
  The module now does file-drops only; Steam config lives in its own
  `programs.steam.config` namespace, where it belongs.

  fh5-vkd3d-no-hvv (which existed only to set VKD3D_CONFIG) collapses
  into the FH5 launchOptions block in hosts/yarn/default.nix.

- `unitConfig.X-ConfigHash` on game-mods.service is replaced with
  `restartTriggers`. NixOS already emits `X-Restart-Triggers=<hash>`
  on the unit; the workaround was redundant. The Type=oneshot,
  RemainAfterExit=no semantics make `systemctl restart` re-run
  ExecStart cleanly on hash change.

- The awk pipeline that patched OptiScaler's stock OptiScaler.ini at
  build time is replaced with a hand-written hosts/yarn/optiscaler-
  fh5-rdna3.ini containing only the keys we override (5 of them).
  OptiScaler's Config::readString defaults missing keys to "auto"
  (Config.cpp:1568), so a minimal file is sufficient. Side benefits:
  one upstream-source dependency removed, a key-rename in upstream
  becomes a behavior change rather than a silent awk-no-match.

  Override values + sources:
    Fsr4Update=true              FH5 wiki, FSR4 Linux Setup
    DlssReactiveMaskBias=0.65    FH5 wiki, "Known Issues"
    FsrNonLinearColorSpace=true  FSR4 wiki, "Image Quality"
    EnableFsr2Inputs=false       FH5 wiki, "Known Issues"
    Dxgi=false                   FH5 wiki

- forza-trigger's three custom Python derivations (pydualsense,
  hidapi-usb, fdp) factored out of default.nix into a sibling
  python-packages.nix. Same logic, single-purpose file. Bumping a
  version is now a one-place hash roll.

- pkgs.dualsensectl removed from the daemon's environment.system-
  Packages. Single-shot writes from the CLI get clobbered by the BG
  sendReport thread within ~4ms anyway, so the tool is only useful
  with the daemon stopped — not worth the unconditional install.
  Bring it in ad-hoc with `nix-shell -p dualsensectl`.

flake.nix

@@ -82,6 +82,10 @@
       url = "github:ChrisOboe/json2steamshortcut";
       inputs.nixpkgs.follows = "nixpkgs";
     };
+    steam-config-nix = {
+      url = "github:different-name/steam-config-nix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
     # Google's official agent-skills for Android development (Apache 2.0).
     # Consumed by home/progs/pi.nix and exposed under ~/.omp/agent/skills/.
     android-skills = {
@@ -238,6 +242,7 @@
               }
             )
             { nixpkgs.overlays = [ (import ./lib/overlays.nix) ]; }
+            inputs.steam-config-nix.nixosModules.default
             ./hosts/${hostname}/default.nix
           ];
         };