feat(two-thumb): typing-insight overlay polish + join cue

[AsafMah]

Builds on the existing gesture-debug overlay (PREF_GESTURE_DEBUG_DRAW_POINTS, relabeled "Typing insight") and makes it part of the experience rather than a debug dump:

• Visual polish — rounded trail caps + a soft glow under the stroke so it reads as a line, not loose dots.
• Join cue — when a gesture/tap extends the current word (fires on extendExistingCompose/usedMergedTrail, the common swipe-fragment combine), a magenta ring lands on the joining fragment's end point + a subtle haptic tick.
• Trail clearing fixed — clears on a fresh-word start (works in manual spacing, not just on autospace) and on backspace.
• Removed the redundant post-commit result pill (the chosen word is already visible in the field).

Rebased onto dev (post-#47, which removed live-converge); the join cue uses the multi-part usedMergedTrail/extendExistingCompose signals that survived. Compiles clean (compileOfflineDebugJavaWithJavac).

First on-device-iterated cut — colors/sizes/haptic are first-pass and easy to tune; dogfooded on a Galaxy S24+. Base: dev.