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]>