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]>