refactor: 1-list turn order model (turnOrderIds === participants.map(id))

Single source of truth. No re-sort after startEncounter. Drag overrides
initiative (cross-init drag allowed, DM choice). Display === rotation by
construction — same array.

shared/turn.js:
- syncTurnOrder(participants) helper: turnOrderIds = participants.map(id)
- startEncounter: sort ALL participants by init (active+inactive), inactive
  stay in slot, nextTurn skips them. currentTurn = first active.
- addParticipant: splice into participants[] by init pos, sync turnOrderIds.
  computeTurnOrderAfterAddition returns insertAt (caller splices + syncs).
- removeParticipant: filter participants[], sync turnOrderIds, advance
  current if removed==current.
- toggleParticipantActive: stay in slot (flip isActive only), sync. Advance
  current only if deact hits current.
- reorderParticipants: cross-init drag allowed (remove same-init restriction).
  Splice participants[], sync turnOrderIds. Fixes BUG-6.
- computeTurnOrderAfterRemoval: only handles current-advance now (list sync
  at call site).

Tests updated to 1-list contract:
- turn.invariant.test.js: 10 tests, turnOrderIds===participants.map(id)
  always, cross-init drag, inactive-in-slot, rotation follows list.
- turn.characterization/reorder/round-rotation/undo/remove: updated
  expectations (inactive-in-slot, cross-init drag, turnOrderIds sync on
  reorder, insertAt return).

Results: shared 90 green. 500-round replay CLEAN (0 skips, 0 doubles,
0 order shifts). BUG-6 (reorder divergence) fixed structurally.

FE App.js still has duplicate turn funcs + sortParticipantsByInitiative
display render (step 4: delete dups, render participants[] directly).

shared/tests/turn.remove.test.js

@@ -29,16 +29,17 @@ describe('removeParticipant turn-order edges', () => {
     expect(e.currentTurnParticipantId).toBe('a');
   });
 
-  test('removing current when all others inactive → current null (BUG-9 candidate)', () => {
+  test('removing current when all others inactive → no active, isStarted stays (BUG-9 candidate)', () => {
     let e = enc([p('a',20),p('b',15),p('c',10)]);
-    e = { ...e, ...startEncounter(e).patch };
-    // deactivate b + c
+    e = { ...e, ...startEncounter(e).patch }; // [a,b,c], cur=a
+    // deactivate b + c (stay in slot, inactive)
     e = { ...e, ...toggleParticipantActive(e, 'b').patch };
     e = { ...e, ...toggleParticipantActive(e, 'c').patch };
     // remove current a
     e = { ...e, ...removeParticipant(e, 'a').patch };
+    // 1-list: turnOrderIds=[b,c], no active → current null, isStarted stays true
+    expect(e.turnOrderIds).toEqual(['b', 'c']);
     expect(e.currentTurnParticipantId).toBeNull();
-    expect(e.turnOrderIds).toEqual([]);
     // isStarted still true but no turn → nextTurn throws (stale state)
   });