Add window-curvature: adjustable curvature per dashboard window

steamFrame.dashboard.windowCurvature: the "Toggle Curvature" row of a
window's More Options menu becomes a control (click: curved -> flat,
flat -> stock; drag up/down with the laser: curvature relative to
SteamVR's stock curve, snapping to 0 and 1). The patch moves each frame's
curvature-origin node to stock distance / value in outgoing scene graphs;
per-window values live in the systemui page. Lookups by signature
("window-curvature" in signatures.json, incl. check-only anchors).

Shared method hooks (lib/hooks.js, passed by mkPatch as 4th argument):
one SendMessage wrapper for dashboard-windows (now VERSION 3) and
window-curvature instead of a chain of per-patch wrappers; legacy
wrappers on top are unwound.

template/home.nix

@@ -39,6 +39,9 @@
   #     # X button on the Steam window: back to the previous window, or
   #     # just the dashboard bar:
   #     steamCloseButton.enable = true;
+  #     # Curvature per window: click the "Toggle Curvature" row of a
+  #     # window's More Options menu to toggle, drag up/down to adjust:
+  #     windowCurvature.enable = true;
   #   };
   #   firefox.enable = true;             # launcher for the Firefox Flatpak
   #   # Hidden from the "+" menu (with Steam Developer Mode on):