docs: README as overview + options, one docs page per feature

README keeps intro, feature list linking docs/, install, two sessions,
usage, the complete options table, changes outside Nix, rollback,
uninstall. Each feature's details (problem, what you get, configuration,
limitations, how it works) move to its own page in docs/; docs/dashboard.md
is split per feature and docs/changes-outside-nix.md becomes
docs/cleanup.md.

docs/dashboard-windows.md

@@ -0,0 +1,61 @@
+# Dashboard windows
+
+`dashboard.windows.*`, module `dashboard-windows` ([options](../README.md#options)).
+
+A [SteamVR dashboard patch](ui-patches.md#steamvr-dashboard-patches).
+
+## Problem
+
+SteamVR dashboard windows can only be enlarged to 2x, and grabbed windows
+pushed back only to 5 m (6 m in theater), too close for a big screen.
+
+## What you get
+
+Higher limits (`null` keeps stock):
+
+| Option | Stock |
+|---|---|
+| `maxScale` | 2 (relative to the window's default size; the theater screen's default is 2.8x larger) |
+| `distance.world.{min,max}` | 0.25-5 m |
+| `distance.theater.{min,max}` | 1-6 m |
+| `distance.dashboard.{min,max}` | 0.3-4 m |
+
+Distances limit pulling in / pushing back a grabbed window (thumbstick or
+scroll while dragging). Changes apply immediately, and turning options off
+reverts on the next switch. The keyboard's range is not patched.
+
+## Configuration
+
+```nix
+steamFrame.dashboard.windows = {
+  maxScale = 4.0;              # resize up to 4x (theater: 11.2x)
+  distance.world.max = 10.0;   # push windows back up to 10 m
+  distance.theater.max = 12.0;
+};
+```
+
+## Caveats
+
+If SteamVR changes its stock grab distances, the distance options silently
+do nothing.
+
+## How it works
+
+`systemui` sends its scene graph to vrcompositor via
+`mailbox.SendMessage("vrcompositor_systemlayer", { type: "update_scene_graph", scene_graph })`,
+and vrcompositor enforces the limits in it:
+
+- `frame-resize-scale-min` / `-max` on window frames;
+- `min-distance` / `max-distance` on grab nodes: `grab-scale` for world
+  windows, `grab-transform` for the theater screen and the dashboard (the
+  keyboard's `grab-transform`, 0.2 / 1 m, is left alone).
+
+They are literals in the bundle, so the patch hooks `SendMessage` on the
+mailbox prototype ([shared hooks](ui-patches.md#shared-method-hooks), shared
+with [window curvature](window-curvature.md)) and edits outgoing scene graphs
+in place. Grab nodes are matched by type plus exact stock values, which is
+why a change of them makes the distance rewrite a no-op. A scene-graph
+resend applies new limits at once.
+
+State: `window.__sfuiDashboardWindows` in the `systemui` page (counters in
+`.hits`).