tests: undo roundtrip (10 green) + reorderParticipants BUG-7 candidate

turn.undo.test.js: every op with log.undo roundtrips to prior state.
startEncounter, nextTurn, togglePause, applyHpChange, toggleCondition,
toggleParticipantActive, addParticipant, removeParticipant, endEncounter.

Found: reorderParticipants returns log:null. Cannot undo. Documents as
BUG-7 candidate (test green now, asserts current behavior).

shared/tests/turn.undo.test.js

@@ -0,0 +1,123 @@
+// Undo roundtrip: every op that returns log.undo must restore prior state.
+// Apply op → patch → apply undo → assert deepEqual original.
+
+const shared = require('@ttrpg/shared');
+const {
+  makeParticipant, startEncounter, nextTurn, togglePause,
+  addParticipant, removeParticipant, toggleParticipantActive,
+  applyHpChange, toggleCondition, reorderParticipants, endEncounter,
+} = shared;
+
+function p(id, init, extra = {}) {
+  return makeParticipant({
+    id, name: id, type: 'monster',
+    initiative: init, maxHp: 100, currentHp: 100,
+    ...extra,
+  });
+}
+function enc(ps) {
+  return { name:'t', participants:ps, isStarted:false, isPaused:false,
+    round:0, currentTurnParticipantId:null, turnOrderIds:[] };
+}
+const snap = (e) => JSON.parse(JSON.stringify(e));
+
+describe('undo roundtrip', () => {
+  test('startEncounter undo restores pre-start', () => {
+    const before = enc([p('a',10),p('b',20)]);
+    const r = startEncounter(before);
+    expect(r.log.undo).toBeTruthy();
+    const after = { ...before, ...r.patch, ...r.log.undo };
+    expect(snap(after)).toEqual(snap(before));
+  });
+
+  test('nextTurn undo restores prior currentTurn/round', () => {
+    let e = enc([p('a',10),p('b',20),p('c',5)]);
+    e = { ...e, ...startEncounter(e).patch };
+    const before = snap(e);
+    const r = nextTurn(e);
+    expect(r.log.undo).toBeTruthy();
+    const after = { ...e, ...r.patch, ...r.log.undo };
+    expect(snap(after)).toEqual(before);
+  });
+
+  test('togglePause undo restores prior paused state', () => {
+    let e = enc([p('a',10),p('b',20)]);
+    e = { ...e, ...startEncounter(e).patch };
+    const before = snap(e);
+    const r = togglePause(e);
+    expect(r.log.undo).toBeTruthy();
+    const after = { ...e, ...r.patch, ...r.log.undo };
+    expect(snap(after)).toEqual(before);
+  });
+
+  test('applyHpChange undo restores prior participants', () => {
+    let e = enc([p('a',10,{maxHp:100,currentHp:100}),p('b',20)]);
+    e = { ...e, ...startEncounter(e).patch };
+    const before = snap(e);
+    const r = applyHpChange(e, 'a', 'damage', 20);
+    expect(r.log.undo).toBeTruthy();
+    const after = { ...e, ...r.patch, ...r.log.undo };
+    expect(snap(after)).toEqual(before);
+  });
+
+  test('toggleCondition undo restores prior participants', () => {
+    let e = enc([p('a',10),p('b',20)]);
+    e = { ...e, ...startEncounter(e).patch };
+    const before = snap(e);
+    const r = toggleCondition(e, 'a', 'stunned');
+    expect(r.log.undo).toBeTruthy();
+    const after = { ...e, ...r.patch, ...r.log.undo };
+    expect(snap(after)).toEqual(before);
+  });
+
+  test('toggleParticipantActive undo restores prior participants + turn order', () => {
+    let e = enc([p('a',10),p('b',20),p('c',5)]);
+    e = { ...e, ...startEncounter(e).patch };
+    const before = snap(e);
+    const r = toggleParticipantActive(e, 'b');
+    expect(r.log.undo).toBeTruthy();
+    const after = { ...e, ...r.patch, ...r.log.undo };
+    expect(snap(after)).toEqual(before);
+  });
+
+  test('addParticipant undo restores prior participants', () => {
+    let e = enc([p('a',10),p('b',20)]);
+    e = { ...e, ...startEncounter(e).patch };
+    const before = snap(e);
+    const np = makeParticipant({ id:'z', name:'z', type:'monster', initiative:15, maxHp:50, currentHp:50 });
+    const r = addParticipant(e, np);
+    expect(r.log.undo).toBeTruthy();
+    const after = { ...e, ...r.patch, ...r.log.undo };
+    expect(snap(after)).toEqual(before);
+  });
+
+  test('removeParticipant undo restores prior participants + turn order', () => {
+    let e = enc([p('a',10),p('b',20),p('c',5)]);
+    e = { ...e, ...startEncounter(e).patch };
+    const before = snap(e);
+    const r = removeParticipant(e, 'b');
+    expect(r.log.undo).toBeTruthy();
+    const after = { ...e, ...r.patch, ...r.log.undo };
+    expect(snap(after)).toEqual(before);
+  });
+
+  test('endEncounter undo restores prior state', () => {
+    let e = enc([p('a',10),p('b',20)]);
+    e = { ...e, ...startEncounter(e).patch };
+    const before = snap(e);
+    const r = endEncounter(e);
+    expect(r.log.undo).toBeTruthy();
+    const after = { ...e, ...r.patch, ...r.log.undo };
+    expect(snap(after)).toEqual(before);
+  });
+
+  test('reorderParticipants has no undo (log: null) — BUG candidate', () => {
+    const ps = [p('a',10),p('b',20),p('c',20)]; // b,c tie
+    let e = enc(ps);
+    e = { ...e, ...startEncounter(e).patch };
+    const r = reorderParticipants(e, 'c', 'b');
+    // Documents: reorderParticipants returns log: null. Cannot undo.
+    // If undo expected here, this is BUG-7.
+    expect(r.log).toBeNull();
+  });
+});