VR keyboard swipe with two controllers: the path from the controller bridge

With both lasers on the keyboard SteamVR forwards per poll only one laser's
movement, so the pressing one may get no touchmoves and its swipe became a
tap. keyboard.vr.swipe.twoHanded (default on; turns on the controller
bridge): a press is attributed to the hand whose laser hit is nearest to it
(within 30 px, the trigger as a tiebreak; without recent frames the first
frame after the press decides, within 60 px). The path comes from runs of
one source: the touch's own moves while they flow, that hand's laser hit
per bridge frame once they pause for 60 ms, touchmoves again if frames
pause. A touchend disagreeing with the laser during a bridge run ends at
the laser. Touchmoves on the other hand's laser move the attribution.
Without fresh frames: touch events only, as before. A second press during
a swipe stays Steam's tap (one text field, one word at a time).

The bridge sends full rate only on demand now: hub.js demand(ms) goes
back through the relay (CDP binding __sfuiCtlIn) to systemui's
__sfuiCtl.demand; the swipe patch asks from the press to the release.

Tests: gesture-input with the two-controller recording and synthetic bridge
frames (a pressing hand without touchmoves, attribution with both lasers on
the keyboard, a stale bridge, frames stopping mid-swipe, re-attribution,
interleaved hover and presses); hub demand.

README.md

@@ -241,6 +241,7 @@ which: [Repository layout](docs/development.md).
 | `steamFrame.keyboard.vr.extraKeys.enable` | bool | `false` | VR keyboard with Esc/Ctrl/Alt, arrows, real chords, AltGr/non-ASCII. |
 | `steamFrame.keyboard.vr.enable` | bool | `false` | Swipe typing, suggestions and Backspace drag on the VR keyboard; the sub-features below are on by default, see [VR keyboard](docs/keyboard.md#swipe-and-suggestions). |
 | `steamFrame.keyboard.vr.swipe.enable` | bool | `true` | Swipe typing. |
+| `steamFrame.keyboard.vr.swipe.twoHanded` | bool | `true` | Swipes also with both lasers on the keyboard: the path from the pressing controller's pose (the controller bridge, sampling during a press while the keyboard is shown). |
 | `steamFrame.keyboard.vr.dictionary.languages` | list of submodules | layout language + English | `{ language; hunspell; words; frequencyOffset; keepFrequentAbove; }`: wordfreq language, `pkgs.hunspellDicts` name (or `null`), most frequent words taken, zipf offset, keep words Hunspell rejects from this zipf on (default `4.0`). Default: the `keyboard.layout` language (de, fr, es, it, nl, pt, sv; 60000) + English (40000, `-0.3`), else English (60000). |
 | `steamFrame.keyboard.vr.dictionary.contractions` | bool | `true` | Words with apostrophes (`couldn't`, `geht's`), swiped by their letters. |
 | `steamFrame.keyboard.vr.dictionary.extraWords` | list of str | `[ ]` | Words always included, casing as given. |