Reveal a screen's controls when a laser lands on them

Hidden controls were taken out of SteamVR, so only ft-screens' own
proximity test could bring them back, and that test didn't match every
controller's laser. They now stay in place fully transparent while
hidden, and SteamVR's hover event on one reveals them, for any laser.

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

docs/reference.md

@@ -35,7 +35,7 @@ Every screen is an overlay named `frametop.screen.N` with four controls:
 - `.roll` rolls the screen when you drag it sideways, like a knob. It snaps level within 2.5°, and scrolling on it turns 5° per notch.
 - `.resize`, the tab on the bottom right corner, sets the width. Screens go down to 15 cm wide.
 
-The controls are sized from both the screen's width and its distance from you, follow the surface of a curved screen, and stay invisible until a laser or the 3D mouse's cursor comes within about 1.5 times a button's size of one. They're translucent until a laser is on them, like SteamVR's own window controls.
+The controls are sized from both the screen's width and its distance from you, follow the surface of a curved screen, and stay invisible until a laser or the 3D mouse's cursor lands on one or comes within about 1.5 times a button's size of it. While invisible they're still there, fully transparent, so SteamVR's laser can find them. They're translucent until a laser is on them, like SteamVR's own window controls.
 
 To pin a screen to a wrist, carry it by its bar and sweep the laser across your other controller. A ring around that controller marks the target, and a dot shows where the laser passes. Crossing the ring arms the pin, and the ring and bar turn blue; crossing it again disarms it. When you let go while armed, the screen rides on that controller at the size, distance, and angle it had, so you can arm the pin first and then turn the screen the way you want. Grab a pinned screen's bar to adjust it; it goes back to the same wrist when you let go unless you disarm it. A pinned screen shows only while you're looking at its front, within the wrist angle, and fades out over the last 10°.