Add cutsceneAppend/cutsceneAppendNext for mutating a running cutscene

cutsceneAppend adds items to the end of a cutscene_t's item array;
cutsceneAppendNext inserts them at a given index instead (e.g.
CUTSCENE_SYSTEM.currentItem + 1, to run immediately after the
currently-executing item), shifting everything from that index
onward to make room. Both are capacity-bounded against the caller-
supplied itemsMax, matching cutsceneParseDoc's convention of never
assuming ownership of the backing buffer's size.

cutscene_t.items drops its const qualifier to allow this in-place
mutation; cutsceneitem_t backing arrays in the test fixtures that get
assigned into a mutable cutscene_t.items are no longer declared const
either, since a const source would otherwise silently violate the
field's new type.

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

test/rpg/entity/test_entityinteract.c

@@ -11,7 +11,7 @@
 #include "ui/focus/uifocus.h"
 #include "rpg/cutscene/cutscenesystem.h"
 
-static const cutsceneitem_t CUTSCENE_TEST_INTERACT_ITEMS[] = {
+static cutsceneitem_t CUTSCENE_TEST_INTERACT_ITEMS[] = {
   { .type = CUTSCENE_ITEM_TYPE_WAIT, .wait = 1.0f }
 };
 static const cutscene_t CUTSCENE_TEST_INTERACT = {