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.

template/home.nix

@@ -25,6 +25,13 @@
   #     closeOnLaunch = true;
   #     launchDebounce = 10;
   #   };
+  #   # SteamVR dashboard windows: resizable up to 4x (stock 2x), pushed
+  #   # back up to 10 m in the world / 12 m in theater mode (stock 5 / 6 m):
+  #   dashboard = {
+  #     windowMaxScale = 4.0;
+  #     windowDistance.world.max = 10.0;
+  #     windowDistance.theater.max = 12.0;
+  #   };
   #   firefox.enable = true;             # launcher for the Firefox Flatpak
   #   # Hidden from the "+" menu (with Steam Developer Mode on):
   #   hiddenApps = [ "lxterminal" "cmake-gui" "firewall-config" "renderdoc" ];