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>

session/frametop.conf.example

@@ -46,7 +46,8 @@ POINTER_HANDS=1            # 1 = pinches click (on the release; hold and move to
 POINTER_PINCH_GAIN=0.5     # hands: the pointer moves this times the pinching hand's angle (precision)
 POINTER_PINCH_DEADZONE=1.5 # hands: degrees the pinching hand moves before the pointer does (a tap's jitter)
 POINTER_GRIP_GAIN=1        # hands: the pointer moves this times the gripping hand's angle
-POINTER_GRIP_BELOW=0.3     # hands: grips beginning more than this (m) below the eyes are ignored (hands on a desk)
+POINTER_GRIP_BELOW=0.35    # hands: grips beginning more than this (m) below the eyes are ignored (hands on a desk)
+POINTER_PINCH_TYPING=1     # hands: no pinch within this many seconds of a key (typing touches thumb to index)
 META_DASHBOARD=0   # 1 = a Meta tap on a pass-through keyboard toggles the SteamVR dashboard (pointer mode only)
 SHARE_KEYS=0       # 1 = keys of keyboards grabbed for the desktop also go to @frametop_keys, for hotkey tools (any local process can listen)
 DISPLAY_OFF_MIN=0  # minutes without use before frametop-power turns the displays off, even if the headset seems worn (0 = never)