Feature/fix scene reset

[malisimat]

Make scene resetting (after last loop is ditched) reliable. Ref issue #97

[Copilot]

Pull request overview

Fixes unreliable scene reset (issue #97) so that ditching the last loop consistently resets quantisation regardless of trigger source. Previously only the keyboard OnAction path checked for ditch-to-zero; the MIDI and serial dispatch paths neither cleared _isSceneReset on ACTIVATE nor invoked the reset check on DITCH, leaving the scene's master length/grain stuck.

Changes:

• Extract the post-action "reset scene if no takes remain" logic into a new _ResetIfEmpty() helper.
• Apply the activate/ditch bookkeeping (clear _isSceneReset, set MIDI quantisation grain on activate, trigger _ResetIfEmpty() on ditch) in _DispatchMidiTriggerEvent and _PumpSerial.
• Add gtest coverage for key/MIDI/serial ditch-to-empty paths and a debounced-ditch-via-OnTick case; expose IsSceneResetForTest() on TestScene.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
JammaLib/src/engine/Scene.h	Declares the new _ResetIfEmpty() helper.
JammaLib/src/engine/Scene.cpp	Replaces inline check in OnAction with the helper and mirrors the activate/ditch handling in MIDI and serial dispatch.
test/JammaLib_Tests/src/engine/Trigger_Tests.cpp	Adds SceneReset tests for key, MIDI and serial trigger paths, plus a debounced-ditch case; adds IsSceneResetForTest() accessor.
.gitignore	Re-groups .copilot/* ignore with an allowlist for .copilot/plan.md.