fix: don't let a despawned final-battle guardian consume its wave

CellWatch counts a final-battle wave as spent when the guardian
spawns, and stops spawning entirely once wFinal reaches 3. If that
guardian despawns without sinking — wrecking teleports the player
beyond the 120 px despawn range — no wave ever spawns again and the
game becomes unwinnable. Give the wave back on despawn.

Found by a full end-to-end campaign playthrough: wrecked during the
finale, and the victory state could no longer be reached.

Author: splch

src/combat.asm

@@ -695,6 +695,19 @@ UpdateEnemy:
     ld [wEnemyActive], a
     ld [wBallEActive], a
     ld [wShadowOAM + 4], a
+    ; a despawned final-battle guardian must not consume its wave (e.g.
+    ; the player wrecked and respawned far away): hand it back so
+    ; CellWatch spawns it again — otherwise the finale can never be won.
+    ld a, [wIsGuardian]
+    and a
+    ret z
+    ld a, [wFinal]
+    and a
+    ret z
+    cp 4
+    ret z
+    dec a
+    ld [wFinal], a
     ret
 
 ; helper: hl += sign-extend(a). clobbers a, h, l