refactor(turn): remove dead computeTurnOrderAfterAddition

Zero call sites. Old turn-array addition logic, superseded by 1-list
slotIndexForInit model. Removed func, export, stale comment, and 3
characterization tests that only locked the dead func's behavior.

shared/tests/turn.characterization.test.js

@@ -6,7 +6,6 @@ const shared = require('@ttrpg/shared');
 const {
   sortParticipantsByInitiative,
   computeTurnOrderAfterRemoval,
-  computeTurnOrderAfterAddition,
   startEncounter,
   nextTurn,
   togglePause,
@@ -326,26 +325,6 @@ describe('computeTurnOrderAfterRemoval', () => {
   });
 });
 
-describe('computeTurnOrderAfterAddition', () => {
-  test('not started = empty', () => {
-    const out = computeTurnOrderAfterAddition(enc([]), 'a');
-    expect(out).toEqual({});
-  });
-
-  test('returns insertAt (1-list: caller splices + syncs)', () => {
-    const e = enc([p('a',3)], { isStarted: true, turnOrderIds: ['a'], participants: [p('a',3)] });
-    const out = computeTurnOrderAfterAddition(e, 'a');
-    // already present → no-op
-    expect(out).toEqual({});
-  });
-
-  test('no-op if already present', () => {
-    const e = enc([], { isStarted: true, turnOrderIds: ['a', 'b'] });
-    const out = computeTurnOrderAfterAddition(e, 'a');
-    expect(out).toEqual({});
-  });
-});
-
 describe('addParticipant', () => {
   test('appends participant', () => {
     const np = p('z', 7);