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]>

assets/battle/moves.jsonc

@@ -1,17 +1,10 @@
 {
-  // Keyed by move id. "ability" matches a BATTLE_ABILITY_* enum name
-  // (see src/dusk/rpg/battle/battleability.h) - there's no JSON loader
-  // consuming this file yet, so field names/shape here are a starting
-  // point, not a contract.
   "moves": {
     "fire": {
       "ability": "FIRE",
       "power": 12,
       "mpCost": 4,
       "target": "SINGLE_ENEMY",
-      // Optional - name of a cutscene (see assets/cutscenes/) to play
-      // when this move is used, resolved lazily like a CUTSCENE item's
-      // "name" field. Omit for a move with no special presentation.
       "cutscene": "battle/fire"
     }
   }