Fix two-thumb backspace fragment modes

[AsafMah]

Summary

• Rename the fragment option to Delete last fragment.
• Fix live fragment backspace so a swipe fragment is deleted as one unit.
• Preserve committed gesture fragment metadata so repeated Backspace after delayed autospace pops one fragment at a time.
• Make whole-word backspace with Delete live composing text off fall back to normal character deletion instead of no-op/double-delete behavior.

Tests

• .\gradlew.bat :app:testOfflineDebugUnitTest --tests helium314.keyboard.latin.InputLogicTest.wholeWordBackspaceDeletesManualSpacingComposingWord --tests helium314.keyboard.latin.InputLogicTest.wholeWordBackspaceWithLiveComposingDeleteOffFallsBackToOneCharacter --tests helium314.keyboard.latin.InputLogicTest.fragmentBackspaceDeletesOnlySwipeFragment --tests helium314.keyboard.latin.InputLogicTest.fragmentBackspaceDeletesLastSwipeFragmentInMultipartWord --tests helium314.keyboard.latin.InputLogicTest.fragmentBackspaceDeletesLastSwipeFragmentAfterAutospaceCommit --tests helium314.keyboard.latin.InputLogicTest.wholeWordBackspaceWithLiveComposingDeleteOnClearsComposingSpan --tests helium314.keyboard.settings.SettingsContainerTest.twoThumbFragmentBackspaceLabelMatchesBehavior --no-daemon
• .\gradlew.bat :app:installStandardDebug --no-daemon installed 1-LeanType_3.8.0-standard-debug.apk on SM-S936B after re-pairing wireless ADB.

Manual Tests — Two-thumb Backspace Modes

#	Steps	Expected Result
1	Open Settings → Two-thumb typing → Backspace behavior.	The fragment option is labeled Delete last fragment.
2	Enable manual spacing, choose Delete last fragment, swipe one word, then press Backspace.	The whole swiped fragment is removed in one press.
3	With Delete last fragment, swipe a word fragment and then swipe another fragment to extend it, then press Backspace.	Only the latest fragment is removed; the previous fragment remains composing.
4	With Delete last fragment, wait for delayed autospace to commit the combined word, then press Backspace twice.	The first press removes the autospace and latest fragment; the second press removes the previous fragment.
5	Choose Delete whole word, turn Delete live composing text off, type letters, then press Backspace.	One character is deleted and the remaining text stays live/composing.
6	Choose Delete whole word, turn Delete live composing text on, type letters, then press Backspace.	The whole live composing word is removed in one press, and the next typed word deletes normally.