tests: reorderParticipants characterization + BUG-6 RED

turn.reorder.test.js: 4 green (swaps, throws-diff-init, throws-missing-id,
documents current no-turnOrderIds-touch) + 1 RED (BUG-6: should update
turnOrderIds to reflect new order).

Found: reorderParticipants changes participants[] array but not turnOrderIds.
nextTurn rotates via turnOrderIds only → mid-combat drag-drop = no effect.
replay-combat.js calls with wrong signature (swallowed by try/catch), so
real path never exercised either.

TODO: BUG-6 added.

TODO.md

@@ -79,6 +79,18 @@
   sequence exactly (damage, heal, conditions, toggleActive, deathSave,
   remove, add, edit, pause/resume, reorder, revive-between-rounds).
 
+### BUG-6: reorderParticipants doesn't update turnOrderIds
+- Test: `shared/tests/turn.reorder.test.js` 'reorder updates turnOrderIds' (RED).
+- `reorderParticipants(enc, draggedId, targetId)` swaps two same-initiative
+  participants in `participants[]` array but leaves `turnOrderIds` unchanged.
+- nextTurn rotates via `turnOrderIds` only → reorder has NO effect on combat
+  rotation. Mid-encounter drag-drop = pointless.
+- replay-combat.js calls reorderParticipants with WRONG signature
+  `(enc, reorderedArray)` — swallowed by try/catch, silent no-op. So
+  replay never exercised real path either.
+- Fix: reorder must also update turnOrderIds to match new participant order
+  (within same-initiative tie).
+
 ## Pipeline
 - [ ] Red test: dead participant still in turnOrderIds, turn still advances to them
 - [ ] Fix `shared/turn.js`: don't drop dead from turn order