Fix addParticipants: slot by initiative instead of append

addParticipants (bulk add all characters) appended to list end, ignoring
initiative. Pre-combat list showed random order until start encounter.
Now slots each participant by initiative desc, preserves existing order +
drag ties. Matches addParticipant single-add semantics.

Tests: 5 bulk-add slot cases (turn.bulkadd.test.js).

shared/tests/turn.bulkadd.test.js

@@ -0,0 +1,49 @@
+// addParticipants (bulk) must slot by initiative, not append.
+const shared = require('@ttrpg/shared');
+const { mockCtx } = require('./_helpers');
+const { buildCharacterParticipant, buildMonsterParticipant, addParticipants, makeParticipant } = shared;
+
+function enc(ps = []) {
+  return { name:'t', participants: ps, isStarted: false, isPaused: false,
+    round: 0, currentTurnParticipantId: null, turnOrderIds: [] };
+}
+function p(id, init) {
+  return makeParticipant({ id, name: id, type: 'monster', initiative: init, maxHp: 10, currentHp: 10 });
+}
+
+describe('addParticipants slot order', () => {
+  test('bulk add slots by initiative desc', async () => {
+    const { ctx } = mockCtx();
+    let e = enc([]);
+    e = await addParticipants(e, [p('a', 5), p('b', 20), p('c', 10)], ctx);
+    expect(e.participants.map(x => x.id)).toEqual(['b', 'c', 'a']);
+  });
+
+  test('bulk add preserves existing order + slots new', async () => {
+    const { ctx } = mockCtx();
+    let e = enc([p('a', 20), p('b', 5)]);
+    e = await addParticipants(e, [p('c', 10)], ctx);
+    expect(e.participants.map(x => x.id)).toEqual(['a', 'c', 'b']);
+  });
+
+  test('bulk add same-init appends after existing (stable tie)', async () => {
+    const { ctx } = mockCtx();
+    let e = enc([p('a', 10)]);
+    e = await addParticipants(e, [p('b', 10), p('c', 10)], ctx);
+    expect(e.participants.map(x => x.id)).toEqual(['a', 'b', 'c']);
+  });
+
+  test('bulk add mixed inits into existing list', async () => {
+    const { ctx } = mockCtx();
+    let e = enc([p('a', 15), p('b', 5)]);
+    e = await addParticipants(e, [p('c', 20), p('d', 10), p('e', 15)], ctx);
+    expect(e.participants.map(x => x.id)).toEqual(['c', 'a', 'e', 'd', 'b']);
+  });
+
+  test('bulk add updates turnOrderIds to match', async () => {
+    const { ctx } = mockCtx();
+    let e = enc([]);
+    e = await addParticipants(e, [p('a', 5), p('b', 20)], ctx);
+    expect(e.turnOrderIds).toEqual(['b', 'a']);
+  });
+});