#384 identity_map row not updated when user renames

Description

When a user changes their username, the user collection updates correctly but the identity_map collection retains a stale row keyed by the old username. The orphan has its last_updated field touched at the time of the rename, but its username value is not changed and distinct_ids remains empty.

Reproduction

1. User signs up with username X (creates user.{username: "X"} and identity_map.{username: "X"}).
2. User renames to Y from their profile.
3. Verify:
   • user.find({username: "X"}) → no result
   • user.find({username: "Y"}) → exists, with updatedAt reflecting the rename
   • identity_map.find({username: "X"}) → still exists, with last_updated near the rename time and distinct_ids: []

Concrete instance (2026-05-20)

• _id 6a0d4d57d254acc8f6495b91
• createdAt 2026-05-20T05:57:43.001Z (signed up as yy)
• updatedAt 2026-05-20T05:58:30.720Z (renamed to zz, 47s later)
• Current user.username = zz
• Orphan: identity_map.{username: "yy"} with last_updated: 2026-05-20T05:57:56.230Z, distinct_ids: []

Expected

One of:

• identity_map row is deleted on rename, OR
• identity_map.username is updated to the new value, OR
• The old username is moved into distinct_ids of the row keyed by the new username, so rename history is preserved usefully.

Impact

• identity_map reads return misleading hits for renamed users.
• Downstream joins against user produce orphans.
• Rename history is partially captured (last_updated touched) but not actionable.