Hands: fixes from the second headset test (pinches)

- The palm-down filter is off by default: the user's deliberate pinches,
  hand raised, read 0.90-0.99, like typing.
- Typing is told apart by the keyboard instead: the input relay sends the
  pointer helper "typing" on key presses, and it takes no pinch within
  POINTER_PINCH_TYPING (1 s) of one.
- Grips are still held to hands raised (POINTER_GRIP_BELOW, 0.35 m below
  the eyes); pinches aren't, since the user's own sat 0.35-0.45 m below,
  elbow resting.
- A grip doesn't begin with the thumb on the index tip: that's a pinch
  with the other fingers curled, which was taken for a grip.
- A pinch's point is the index and middle knuckles: the tips' midpoint
  moved 1-2 cm as the pinch opened, dragging every release off its press.
- ft-hands --gesture-log prints what the detectors measure, 10 times a
  second.

Pinches still aren't reliable enough to use; hand tracking is parked for
now in favour of the controllers.

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

input/input-relay.py

@@ -34,6 +34,8 @@ reaches games; see pointer/helper/vrbuttons.h).
 
 In gaze mode outside games, the helper keeps the pointer ("gazeawake 1", repeated every 5
 seconds; "gazeawake 0" or silence ends it): the pointer isn't released when the mouse is idle.
+Typing on a keyboard sends the helper "typing" (at most 4 times a second): it takes no hand
+pinches right after a key, since typing touches thumb to index like a pinch.
 
 Keys also go to ft-screens (@ft_screens, the Frametop desktop's compositor), which
 types them into the desktop screen that has focus: from pass-through keyboards, and
@@ -614,6 +616,7 @@ def main():
     load_config()
     meta_down = False  # Meta pressed with no other key yet: a tap toggles the dashboard
     screens_sock = socket.socket(socket.AF_UNIX, socket.SOCK_DGRAM | socket.SOCK_NONBLOCK)
+    last_typing = 0.0  # the helper was last told of a key then (see "typing" at the top)
 
     def vr_bind(now):
         """Tell the pointer helper which controller buttons to take from games."""
@@ -957,6 +960,9 @@ def main():
                     # Observed only, unless typing goes to the desktop. With META_DASHBOARD=1,
                     # a Meta tap on any keyboard toggles the dashboard.
                     if node.role == "passthrough" and etype == EV_KEY:
+                        if value == 1 and code < BTN_MISC and now - last_typing >= 0.25:
+                            last_typing = now
+                            screens_sock.sendto(b"typing", HELPER)
                         to_screens(code, value)
                         if node.grabbed and code < BTN_MISC and value in (0, 1):
                             share_key(node, code, value)