B7a: Prefix-aware gesture result stripping

[AsafMah]

Part of #97.

A better fake track can make the lib return the whole word (prefix tech + swipe nology → lib returns technology). The current concat then double-counts (techtechnology / garbage).

In InputLogic.onUpdateTailBatchInputCompleted, before concatenating prefix + recognizedSuffix: if the recognized word starts (case-insensitive) with the current composing prefix, strip the prefix; else fall back to the existing seed-letter strip.

Lowest-risk step — do first; likely fixes some "almost worked" cases on its own.

Acceptance: golden-corpus (#21) A/B shows no regression; whole-word returns no longer double-count.

[AsafMah]

This appears to be already built, but stranded on an unmerged branch.

The work lives on b7a-prefix-aware-stripping, which was never merged to dev and — until today — existed on no remote at all. It has now been pushed to origin as a backup, so it is at least safe:

origin/b7a-prefix-aware-stripping (5 commits ahead of dev), including app/src/main/java/helium314/keyboard/latin/inputlogic/IdealPrefixTrailBuilder.java.

Flagging so this isn't rebuilt from scratch. It was identified as key prior art by the two-thumb research in #136, which independently concluded that tap → micro-stroke promotion is the right direction (while falsifying the re-timing approach in #100).

Note the branch also carries uncommitted debug logging in its worktree that was not pushed.