Pointer: read the overlay list every 20 s, not every second

The helper ran `vrcmd --overlays` once a second while the pointer was awake, and in gaze
mode the pointer never sleeps. Each run is a shell plus vrcmd, a new SteamVR client, about
26 to 30 ms of CPU, so about 3% of a core all the time.

The list is now read every 20 seconds, and at once (at most once a second) when it may have
changed: the pointer waking, the dashboard opening or closing or creating an overlay, the
scene app changing, an "overlays" request, and a left click that hit nothing, which may be
on a panel that came up since. The thread waits on a condition variable instead of waking
every 100 ms, so it sleeps while paused. The main loop looks the keys up again as soon as a
new list is in, rather than at its next 1 s tick. Overlays already on the list still show
and hide within 50 ms, from the IsOverlayVisible poll.

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

docs/design.md

@@ -118,7 +118,7 @@ The driver starts disconnected, because holding the right-hand role while SteamV
 
 Mouse motion turns into yaw and pitch around an anchor, the head position at the last recenter. A ray from the anchor is tested against every visible overlay with `ComputeOverlayIntersection`. On a hit, the cursor sits on that surface; otherwise it floats at `POINTER_DISTANCE`. Since the anchor isn't your current eye position, a second test runs along your line of sight to the cursor point, and anything nearer wins, so the cursor always lands on what you see under it. Overlays in `POINTER_IGNORE` are left out of both tests. A display-only panel, like a performance overlay locked to your view, has no input method, so SteamVR's laser passes through it, but `ComputeOverlayIntersection` still hits it, and the cursor stuck to it. The laser starts just before the cursor point, so an ignored panel nearer to you doesn't catch it either.
 
-OpenVR has no call to list other programs' overlays, so the helper runs `vrcmd --overlays` in the background. It includes hidden overlays, because a floating window's controls only appear while something hovers the window, and the cursor has to find them immediately.
+OpenVR has no call to list other programs' overlays, so the helper runs `vrcmd --overlays` in the background. It includes hidden overlays, because a floating window's controls only appear while something hovers the window, and the cursor has to find them immediately. Each run is a shell and a new SteamVR client, about 30 ms of CPU, and it ran every second while the pointer was awake, which in gaze mode is all the time. Now it runs every 20 seconds, and at once when the pointer wakes, when the dashboard opens or closes, when a game starts or ends, and when a left click hits nothing (a panel that came up since). An overlay already on the list showing or hiding needs no new list: the helper checks the visibility of the ones it knows every 50 ms.
 
 The laser starts partway along your line of sight to the cursor rather than at your eye. SteamVR sizes its hit dot by distance from the laser's origin, and a laser from the eye still shows a beam in each eye. Starting it close to the target makes the beam and the dot tiny, while `POINTER_ORIGIN_MARGIN` keeps the origin in front of the small window controls, which float a few centimetres in front of their panels. The helper's own white dot is the visible cursor. In empty space it's an interactive overlay that the laser lands on, so SteamVR never draws a laser into nothing.