fix(coordinator): repoint rotation chores when their assignee is deleted (#787)

[tempus2016]

Closes #787.

async_remove_child stripped the child from assigned_to and (since #786) cleared the swap override, but never touched assignment_current_child_id. ChildStatsSensor includes a non-everyone chore only when that field matches the child — and a deleted id matches nobody, so the chore went invisible to every surviving child until the midnight pass recomputed it.

The completion service, the per-chore button entity and the to-do list all gate on _compute_active_children, which recomputes from the live pool, so they carried on accepting the survivor. Only the card disagreed — the mirror image of #781, where the card showed a chore the backend refused.

Fix

Chores whose pointer names the removed child are repointed at today's live pool via _compute_daily_assignments(), in async_remove_child after the child is out of storage. Recomputed rather than just blanked, per the issue's open question: blanking would leave the chore hidden until midnight anyway, and the daily map is exactly what the midnight pass would have produced. When the pool empties, the pointer clears.

Verification

Reproduced on the live ha-dev instance first — two children, an alternating chore, delete today's assignee:

active   = zz787 Ana
survivor = zz787 Ben
after deleting the assignee:
  assignment_current_child_id = 7fe753256dba4bdf (zz787 Ana)
  points at a deleted child   = True
  assigned_to                 = [743d7f16c9764b9c]      <- correctly stripped
  survivor stats sensor lists it = False                <- the bug
RESULT: FAIL — chore hidden from the survivor

The same probe is re-run against the merged fix on ha-dev.

4 new tests in tests/test_rotation_quota.py, 3 of which fail on main:

• pointer is repointed at a surviving child
• the child sensor still lists the chore for the survivor — the user-visible symptom, via a real ChildStatsSensor
• pointer clears when the pool empties
• removing an off-rotation child leaves the pointer alone (control; passed before the fix too)

1565 tests pass, ruff clean.