Add dashboard-windows (resize/grab-distance limits) and steamvr-debugger

dashboard-windows: steamFrame.dashboard.windowMaxScale and
windowDistance.{world,theater,dashboard}.{min,max}, one runtime patch of the
SteamVR dashboard (systemui, 127.0.0.1:8087) that rewrites
frame-resize-scale-max and grab min/max-distance in the scene graph sent to
vrcompositor, then asks for one resend so changes apply immediately.
Registered only when an option is set. The mailbox class and the
scene-graph scheduler are found by signature (mkPatch, "dashboard-windows"
in lib/signatures.json, bundle vrwebui-systemui), so
scripts/check-signatures.mjs checks them offline too.

steamvr-debugger: steamFrame.steamvrDebugger.enable, defaulting to true when
a UI patch uses port 8087; merges VRWebHelper.DebuggerEnabled into
steamvr.vrsettings before every SteamVR start, and reverts it (only if it
set it) when turned off.

flake.nix

@@ -19,6 +19,8 @@
       hidden-apps = ./modules/hidden-apps.nix;
       steam-ui-patches = ./modules/steam-ui-patches.nix;
       launcher-menu = ./modules/launcher-menu.nix;
+      steamvr-debugger = ./modules/steamvr-debugger.nix;
+      dashboard-windows = ./modules/dashboard-windows.nix;
       # Built with the consumer's pkgs; only the source comes from our input.
       # The key lets the module system deduplicate it when it is imported both
       # directly and through `default`.