Add source-key action target slides

[AsafMah]

Summary

• Add source-key action target config for press-slide-release from the Enter/action key, period, and comma.
• Open a temporary popup target keyboard on slide-out, then dispatch the selected existing toolbar/key-code action on release.
• Default the action key target panel to JOIN_NEXT, FORCE_NEXT_SPACE, and UNDO_WORD.
• Add an Advanced setting for target lists, e.g. ACTION=JOIN_NEXT,FORCE_NEXT_SPACE,UNDO_WORD;PERIOD=UNDO_WORD.
• Add parser/canonicalization tests and a changelog entry.

Notes

• This replaces the closed wrong-direction PR Add configurable source-key swipe actions #105.
• This is slide-to-target selection, not (source + direction) -> action.
• It does not add Enter/period/comma to KeyCode.isModifier().
• Existing Shift/Symbol/Numpad sliding, space swipes, delete slider, and letter glide typing stay on their existing paths.

Verification

• ./gradlew.bat :app:testOfflineDebugUnitTest --tests "helium314.keyboard.latin.utils.SourceKeyActionTargetsTest" --tests "helium314.keyboard.settings.SettingsContainerTest" --no-daemon --console=plain
• ./gradlew.bat :app:assembleStandardDebug --no-daemon --console=plain

Closes #37