The primary focus of this PR is to enable the Web OSK to ensure that gestures are only considered for selection (by the gesture-recognizer engine) if their corresponding key supports that gesture. The abstractions previously put in place toward this end seem sufficient for the task, which is always nice to see. Part of this includes restoring the standard, repeating held-backspace behavior - as that is (1) key-specific and (2) backspace uses special-key characteristics. (It should trigger immediately, not after the key is 'released'.)

The secondary focus is to fix some of the notable bugs that arose as integration's been improving; at this stage, simple-taps and roaming touch appear to have fully stabilized. Roaming touch, in particular, gets most of the love in this regard; the most notable bugs being fixed herein are due to previously-incomplete handling of its supporting aspects within the gesture engine. (The gaps caused some... very interesting side-effects to debug.) Also, the "off-the-top padding" permitted by the OSK in the past gets re-established through use of the corresponding part of the gesture-engine's config abstraction.

Automated tests are still broken at this point. I considered including their fixes in this PR, but the changeset is pretty orthogonal and would add complexity for the review, so I spun it off as #9719. Web's CI build check should pass there, though probably not the filesize check.

@keymanapp-test-bot skip