fix(two-thumb): clear stale stroke buffer so re-typed words don't accumulate geometry (#7)

[SHAWNERZZ]

Fixes #7.

Problem

Building a word from a tap stem + a swipe (e.g. tap do, swipe the rest → doesn't) is the intended multi-part flow, and space stays the word-submission point. But when such a word mis-recognized and you backspaced to retry, the previous stroke geometry stuck around and each retry built an ever-longer garbage word.

Root cause (traced end-to-end)

• The merged stroke lives in WordComposer.mInputPointers. It's intentionally not cleared by reset()/backspace (so a gesture commit can re-feed it via setBatchInputWord), and ResizableIntArray.addAt(index) overwrites in place without shrinking the length.
• So after a word is abandoned via backspace, the raw buffer keeps its length. Re-typing the stem overwrites only the leading indices; the stale tail survives. The next swipe snapshots the whole stale buffer as the merged-trail extend base → an ever-longer word that grows with each retry.
• PR fix(two-thumb): clear stale merged-trail extend-base on delete + gesture lifecycle AsafMah/LeanType#61 cleared the extend-base flag and mLiveStroke, but not the mInputPointers buffer the base is re-armed from — so the leak persisted through that fix.

Fix

Reset mInputPointers when the first tap of a fresh word lands (newIndex == 0 in WordComposer.applyProcessedEvent). This:

• covers every abandonment path (whole-word / batch / per-char-to-empty / cursor move),
• never touches the gesture-commit flow (batch mode is guarded),
• preserves multi-part composition (a tap extending a gesture has newIndex > 0),
• can't disturb mid-word backspace coordinates for normal typing.

Testing

• New WordComposerTest.testFreshWordResetsStaleInputPointers: a stale 5-point buffer surviving reset() is dropped to 1 point on the first fresh tap (fails pre-fix).
• Full :app:testOfflineDebugUnitTest: 243 tests, 12 failed = main baseline (242) + this PR's 1 new test, with the identical 12 pre-existing/environment-dependent failures. Zero new failures; normal-typing/autocorrect/recorrection tests unaffected.
• Validated on-device: re-typing a mis-recognized multi-part word now recognizes cleanly each retry instead of growing; technology/silo-style multi-part and normal typing still work.

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