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.lock

@@ -416,6 +416,27 @@
         "type": "github"
       }
     },
+    "flake-parts_4": {
+      "inputs": {
+        "nixpkgs-lib": [
+          "steam-config-nix",
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1765835352,
+        "narHash": "sha256-XswHlK/Qtjasvhd1nOa1e8MgZ8GS//jBoTqWtrS1Giw=",
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
+        "rev": "a34fae9c08a15ad73f295041fec82323541400a9",
+        "type": "github"
+      },
+      "original": {
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
+        "type": "github"
+      }
+    },
     "flake-utils": {
       "inputs": {
         "systems": "systems_2"
@@ -454,7 +475,7 @@
     },
     "flake-utils_3": {
       "inputs": {
-        "systems": "systems_10"
+        "systems": "systems_11"
       },
       "locked": {
         "lastModified": 1731533236,
@@ -1103,6 +1124,7 @@
         "rust-overlay": "rust-overlay",
         "senior_project-website": "senior_project-website",
         "srvos": "srvos",
+        "steam-config-nix": "steam-config-nix",
         "trackerlist": "trackerlist",
         "vpn-confinement": "vpn-confinement",
         "website": "website",
@@ -1204,6 +1226,28 @@
         "type": "github"
       }
     },
+    "steam-config-nix": {
+      "inputs": {
+        "flake-parts": "flake-parts_4",
+        "nixpkgs": [
+          "nixpkgs"
+        ],
+        "systems": "systems_10"
+      },
+      "locked": {
+        "lastModified": 1773421780,
+        "narHash": "sha256-bkZGvIWV8JavLMQ2KzddrMCf9YBeiIM4OgdiwKcFx8M=",
+        "owner": "different-name",
+        "repo": "steam-config-nix",
+        "rev": "7b8021b2739733c547e2fe02739e6b8452813aa7",
+        "type": "github"
+      },
+      "original": {
+        "owner": "different-name",
+        "repo": "steam-config-nix",
+        "type": "github"
+      }
+    },
     "systems": {
       "locked": {
         "lastModified": 1681028828,
@@ -1234,6 +1278,21 @@
         "type": "github"
       }
     },
+    "systems_11": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
+    },
     "systems_2": {
       "locked": {
         "lastModified": 1681028828,