Rework CUTSCENE_ITEM_TYPE_INSERT to splice into the running scene's item array

Replaces the separate insert buffer/scene and insertStack return-frame
mechanism with cutsceneAppend/cutsceneAppendNext, so an inserted
cutscene's items become part of the running scene's own array instead
of a jump-and-return. Moves the loaded-items buffer/scene from file-scope
globals into cutscenesystem_t, and extracts shared start-cutscene setup
into cutsceneSystemPrepare.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

test/rpg/entity/test_entityinteract.c

@@ -14,9 +14,10 @@
 static cutsceneitem_t CUTSCENE_TEST_INTERACT_ITEMS[] = {
   { .type = CUTSCENE_ITEM_TYPE_WAIT, .wait = 1.0f }
 };
-static const cutscene_t CUTSCENE_TEST_INTERACT = {
+static cutscene_t CUTSCENE_TEST_INTERACT = {
   .items = CUTSCENE_TEST_INTERACT_ITEMS,
   .itemCount = 1,
+  .itemsMax = 1,
   .pause = CUTSCENE_PAUSE_NONE
 };