Drive battle rounds through cutscene items, fix render/init bugs

Adds BATTLE_START, BATTLE_SET_STATE and BATTLE_WAIT_SELECTION cutscene
items so a battle's state transitions (opening -> pre-round -> selection
-> executing) are chained through the running cutscene, with
cutsceneSystemInsertItemsNext/InsertItems splitting "splice" from
"splice and advance immediately". Adds battlestateselection's
get-current-fighter/player-attack/player-flee API backing the battle UI,
and battleMoveGetIdByName for JSON-authored ability lookups.

Fixes: off-by-one in battleMoveInit that let the first-defined move
alias BATTLE_MOVE_ID_NULL; a segfault in scenebattle.c from rendering
unfilled (NULL) fighter slots; stale battleGetCurrentFighter/
battlePlayerAttack/battlePlayerFlee references in the battle UI.

Adds assertions across the battle/scene/UI code for fighter-index
bounds and non-null invariants. Updates battletest.jsonc for the new
BATTLE_START item schema.

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

assets/cutscenes/battletest.jsonc

@@ -1,24 +1,22 @@
 {
   "items": [
     {
-      "type": "START_BATTLE",
-      "encounterType": "REGULAR",
-      "fleeAvailable": true,
+      "type": "BATTLE_START",
       "enemies": [
         {
-          "stats": { "attack": 8, "defense": 4, "magic": 0, "speed": 5, "luck": 2 },
-          "healthMax": 30,
-          "mpMax": 0
+          "stats": {
+            "attack": 8, "defense": 4, "magic": 0, "speed": 5, "luck": 2,
+            "healthMax": 30, "mpMax": 0
+          },
+          "abilities": ["fire"]
         },
         {
-          "stats": { "attack": 6, "defense": 3, "magic": 0, "speed": 7, "luck": 2 },
-          "healthMax": 20,
-          "mpMax": 0
+          "stats": {
+            "attack": 6, "defense": 3, "magic": 0, "speed": 7, "luck": 2,
+            "healthMax": 20, "mpMax": 0
+          }
         }
       ]
-    },
-    {
-      "type": "REGULAR_BATTLE"
     }
   ]
 }