Add a potential hazards doc for the input changes

It lists what can go wrong with the relay taking the volume keys (keymaps
left remapped after a crash, the headset's click button on the same
device, wpctl stepping the default output), with key releases (a key left
held in the desktop when its release never arrives), and with keyboards
grabbed for typing (hotkey tools lose them; SHARE_KEYS is off by default).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -99,7 +99,7 @@ setup/bluetooth/install.sh uninstall   # if you installed the Bluetooth fixes
 
 ## How it works
 
-A Plasma session runs nested inside ft-screens (`screens/`), a small Wayland compositor. KWin opens one window per screen, ft-screens sets each window's size, and each frame goes to SteamVR as an overlay without being copied. An input relay (`input/`) keeps Bluetooth mice working in SteamVR and feeds the mouse to the 3D pointer, which drives a virtual SteamVR controller (`pointer/`). [docs/reference.md](docs/reference.md) covers each piece, and [docs/design.md](docs/design.md) explains the design and what we learned about SteamVR on the Frame.
+A Plasma session runs nested inside ft-screens (`screens/`), a small Wayland compositor. KWin opens one window per screen, ft-screens sets each window's size, and each frame goes to SteamVR as an overlay without being copied. An input relay (`input/`) keeps Bluetooth mice working in SteamVR and feeds the mouse to the 3D pointer, which drives a virtual SteamVR controller (`pointer/`). [docs/reference.md](docs/reference.md) covers each piece, and [docs/design.md](docs/design.md) explains the design and what we learned about SteamVR on the Frame. [docs/hazards.md](docs/hazards.md) lists known ways the input handling can go wrong.
 
 | Folder | What it is |
 | --- | --- |