fix(two-thumb): live-compose reliability - casing, tap-weighting, gesture correction, backspace

[SHAWNERZZ]

Reliability fixes for the existing two-thumb / live-compose feature. Four areas:

Casing

• Sentence start: a fresh-word swipe that begins right at a sentence/field boundary no longer drops the leading capital (yeah → Yeah). When the visual shift indicator hasn't caught up, casing falls back to the authoritative auto-caps state (gated by the auto-cap setting).
• Across re-recognition: the merged-trail re-recognizer replaces the whole composing word with a lowercase lemma; the word's capital (and a deliberate all-caps acronym) is now preserved.
• No over-capitalization: shift-lock is only inferred for genuinely all-caps words of ≥2 letters, so a lone re-composed capital (I, A) gets first-letter shift, not ILL/AND.

Tap weighting

When you tap a prefix then swipe the rest, the tapped letters now carry real weight, so a long swipe can't out-vote them (tap s,h + swipe → should, not would). The prefix geometry is densified by clustering samples at each key center — capped per key, spread symmetrically, and with the already-dense swipe-then-swipe path kept timing-equivalent to before.

Gesture correction

After a multi-part (merged-trail) gesture, the strip keeps the recognizer's own whole-word alternatives, so the intended word stays one tap away when the recognizer ranks the wrong one first (e.g. super when speer won) — instead of being buried behind typed-word suggestions for the auto-committed word. Scoped to the merged-trail case (candidates are whole words, so picking one can't lose part of the word); the true-concatenation path still re-runs typed suggestions.

Backspace / gesture clearing

After editing a word, the next swipe builds the new word cleanly instead of reusing the old gesture — covers delete-part-and-reswipe, fragment-pop, whole-word delete, and select-and-delete-then-retype. Also: fragment boundaries are cleared when the composing word is gone, and an extend gesture that recognizes nothing marks the stroke stale so a later swipe can't build on a doubled buffer.

Notes

• Targets dev per CONTRIBUTING. Compiles standalone on dev; WordComposerTest and InputLogicTest pass.
• Independent of the adaptive-typing PR. One tiny interaction (the gesture-correction strip change should clear the adaptive next-key prior) is intentionally left out here to keep this PR self-contained; it's a one-line follow-up for whenever both this and the adaptive PR are merged.