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.

modules/dashboard-windows.nix

@@ -0,0 +1,101 @@
+# Resize and grab-distance limits of SteamVR dashboard windows (Steam, app
+# windows, overlays, the theater screen, the dashboard itself), through a
+# runtime patch of SteamVR's dashboard page (vrwebhelper "systemui", DevTools
+# 127.0.0.1:8087; see dashboard-windows/patch.js for how it works). It is
+# registered with steam-ui-patches.nix only when an option is set, and turns
+# on the SteamVR web helper debugger (steamvr-debugger.nix), which needs one
+# SteamVR restart the first time. Changes apply immediately (the dashboard
+# resends its scene graph); unsetting all options reverts to stock.
+# Depends on SteamVR UI internals, found by signature (lib/signatures.json,
+# "dashboard-windows"; scripts/check-signatures.mjs checks them offline).
+{ config, lib, pkgs, ... }:
+let
+  cfg = config.steamFrame.dashboard;
+  inherit (lib) mkOption types;
+  inherit (import ./lib { inherit pkgs; }) mkPatch;
+
+  # Stock grab distance ranges (meters), matched by the patch.
+  stockDistance = {
+    world = { min = 0.25; max = 5; };
+    theater = { min = 1; max = 6; };
+    dashboard = { min = 0.3; max = 4; };
+  };
+
+  rangeOption = kind: what: {
+    min = mkOption {
+      type = types.nullOr types.number;
+      default = null;
+      description = ''
+        Closest distance in meters ${what} can be pulled in to while grabbed.
+        null = stock (${toString stockDistance.${kind}.min} m).
+      '';
+    };
+    max = mkOption {
+      type = types.nullOr types.number;
+      default = null;
+      example = stockDistance.${kind}.max * 2;
+      description = ''
+        Farthest distance in meters ${what} can be pushed back to while
+        grabbed. null = stock (${toString stockDistance.${kind}.max} m).
+      '';
+    };
+  };
+
+  effective = kind: {
+    min = if cfg.windowDistance.${kind}.min != null then cfg.windowDistance.${kind}.min else stockDistance.${kind}.min;
+    max = if cfg.windowDistance.${kind}.max != null then cfg.windowDistance.${kind}.max else stockDistance.${kind}.max;
+  };
+
+  distanceSet = lib.any (r: r.min != null || r.max != null) (lib.attrValues cfg.windowDistance);
+  enabled = cfg.windowMaxScale != null || distanceSet;
+in {
+  imports = [ ./steam-ui-patches.nix ./steamvr-debugger.nix ];
+
+  options.steamFrame.dashboard = {
+    windowMaxScale = mkOption {
+      type = types.nullOr types.number;
+      default = null;
+      example = 4.0;
+      description = ''
+        Largest size, relative to their default size, that SteamVR dashboard
+        windows (Steam, app windows, overlays, the theater screen) can be
+        enlarged to with the resize handle. null = stock (2; the smallest is
+        0.25). The theater screen's default size is 2.8x that of a normal
+        window, so its limit is 2.8x this value. Runtime patch of the SteamVR
+        dashboard (steamFrame.uiPatches), applied immediately.
+      '';
+    };
+    windowDistance = {
+      world = rangeOption "world" "windows placed in the world (not attached to the dashboard)";
+      theater = rangeOption "theater" "the theater-mode screen";
+      dashboard = rangeOption "dashboard" "the dashboard itself";
+    };
+  };
+
+  config = lib.mkIf enabled {
+    assertions =
+      [ {
+        assertion = cfg.windowMaxScale == null || cfg.windowMaxScale > 0;
+        message = "steamFrame.dashboard.windowMaxScale must be positive.";
+      } ]
+      ++ map (kind: let r = effective kind; in {
+        assertion = r.min > 0 && r.max >= r.min;
+        message = "steamFrame.dashboard.windowDistance.${kind}: need 0 < min <= max (stock values fill in unset ones), got ${toString r.min}-${toString r.max}.";
+      }) (lib.attrNames stockDistance);
+
+    steamFrame.uiPatches.patches = [ {
+      name = "dashboard-windows";
+      endpoint = "http://127.0.0.1:8087";
+      target.title = "systemui";
+      patch = mkPatch {
+        name = "dashboard-windows";
+        src = ./dashboard-windows/patch.js;
+        opts = {
+          maxScale = cfg.windowMaxScale;
+          distance = cfg.windowDistance;
+        };
+      };
+      unpatch = ./dashboard-windows/unpatch.js;
+    } ];
+  };
+}