engine: timeline panel core edit commands migrate to the facade (part 1)

- new batch primitives: split_clips (link-preserving, single undo
  command), delete_clips (gap replace + optional ripple with explicit
  region support), ripple_delete_range, marker_add_ex with color
- razor/split-at-playhead, clip delete, ripple-to-point, track delete,
  and the non-dialog marker path now issue facade commands instead of
  the app's own undo command classes
- batch operations deliberately produce one undo command per user
  action (deleting twenty clips is one entry, not twenty); selection
  and transition removal stay UI-side as documented leftovers

engine/tests/oakengine_init_test.cpp

@@ -220,7 +220,7 @@ static void test_sequence_and_save_load(void)
 
 	// No markers on a fresh sequence.
 	assert(oakengine_sequence_marker_count(seq) == 0);
-	assert(oakengine_sequence_marker_at(seq, 0, NULL, NULL, 0) ==
+	assert(oakengine_sequence_marker_at(seq, 0, NULL, NULL, 0, NULL) ==
 		   OAKENGINE_E_NOT_FOUND);
 
 	// Save; the modified flag clears and the filename is adopted.
@@ -384,7 +384,7 @@ static void test_null_safety(void)
 	assert(oakengine_sequence_set_workarea(NULL, 0, 0, 0) ==
 		   OAKENGINE_E_INVALID);
 	assert(oakengine_sequence_marker_count(NULL) == 0);
-	assert(oakengine_sequence_marker_at(NULL, 0, NULL, NULL, 0) ==
+	assert(oakengine_sequence_marker_at(NULL, 0, NULL, NULL, 0, NULL) ==
 		   OAKENGINE_E_INVALID);
 }