Find a burst weapon wherever it is on the rack, not just in the first five

endBurst looked its tin up with [0, 1, 2, 3, 4].find(...) — correct for exactly
as long as the rack held five slots. Growing RACK_MAX to 9 meant a burst weapon
picked up into slot 5..9 could never be found, so it was never spent: you fired
it and the count stayed where it was, forever. Oliver hit it with the beacon, the
rocket and the flamer, which together with the gun are precisely the four
kind:'burst' weapons — endBurst is the only spend path that searched for its slot
instead of being handed one.

The serve weapons were never affected, and that is the tell: bomb, triple, fire
and the laser all spend through spendSlot(G.weapon), the live index. Checked the
laser end to end at slot 8 rather than reasoning about it — 6, 5, 4, 3, 2, 1,
gone.

Fixed at the invariant rather than at the call site. The rack has always had a
comment saying everything reads through the slot accessors so nothing else needs
to know how it is stored; slotOf(id) is now the fourth one, and it is the single
place that knows the range. Verified all four burst weapons arm from a real tap
in slots 5-8 and leave the rack when the burst expires.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

gameplay.js

@@ -511,12 +511,20 @@ function startChangeover() {
   // no tins here anymore — weapons are won in the duel, never handed out on a schedule
 }
 
-// ---- the courtside rack. Five slots still, but only slot 0 is fixed: it is BALL (infinite), or a
-// FIFTH gathered weapon riding it temporarily. Slots 1..4 are whatever you have picked up, in order.
-// Everything reads through these three, so nothing else has to know how the rack is stored.
+// ---- the courtside rack. RACK_MAX + 1 slots, and only slot 0 is fixed: it is BALL (infinite), or
+// an overflow weapon riding it temporarily. Slots 1..RACK_MAX are what you have picked up, in order.
+// EVERYTHING READS THROUGH THESE FOUR, so nothing else has to know how the rack is stored — and
+// nothing else may hardcode how MANY slots there are. `slotOf` exists because endBurst did exactly
+// that ("[0,1,2,3,4].find(...)"), so the moment the rack grew past five, a burst weapon sitting in
+// slot 5..9 could never be found and never spent: beacon, flamer and rocket sat at their pickup
+// count forever. One accessor, one place that knows the range.
 export const slotEntry = i => (i === 0 ? G.temp : G.rack[i - 1]) || null;
 export const slotId = i => slotEntry(i)?.id || (i === 0 ? 'ball' : null);
 export const slotStock = i => { const e = slotEntry(i); return e ? e.n : (i === 0 ? Infinity : 0); };
+export const slotOf = id => { // -1 if you are not carrying it
+  for (let i = 0; i <= RACK_MAX; i++) if (slotId(i) === id) return i;
+  return -1;
+};
 export function selectWeapon(i) {
   if (G.interlude) return; // the sudden-death duel is played under tennis rules — plain balls only
   if (slotStock(i) <= 0) return; // dud: nothing in that slot (slot 0 is only ever empty if a spent
@@ -1022,8 +1030,8 @@ export function endBurst() {
   const id = G.gunId;
   G.gunId = null;
   if (!id) return;
-  const slot = [0, 1, 2, 3, 4].find(i => slotId(i) === id);
-  if (slot !== undefined) spendSlot(slot);
+  const slot = slotOf(id);
+  if (slot >= 0) spendSlot(slot);
 }
 
 // The laser fired from a TAP (the keyboard/two-stage path). The drag path solves its own direction