fix(gateway): relay live gear changes to nearby peers

[Mathih13]

Nearby Characters keep stale gear after an equip, swap, unequip or permanent enchant change. The item Relay sends updates only to the owner.

Add a peer Relay through the AOI audience on the same Shard. Each writer checks that its World Session still sees the Character. Peers receive only visible item fields. The first eligible writer reads the current slot occupant and shares that result with the other recipients, so a swap cannot clear replacement gear and each affected slot needs only one item-table scan.

Closes #634.

What you verified

• Gateway suite: 2,143 passed. Four focused tests cover slot contents, enchant ID mapping, swaps, visibility, Transfer and private-field exclusion.
• A disposable SpacetimeDB fixture passed the real Coordinator insert, update and delete callbacks. It checks enchant and Random Property changes, unequip, re-equip and deletion. CI now runs this test.
• cargo fmt --all -- --check and workspace Clippy with all targets, all features and -D warnings passed.
• Independent adversarial Standards and Spec reviews. Fixed the repeated table scans and added callback integration coverage. No remaining blockers.

What you did NOT verify

No real build-5875 client or production Realm check. The packet tests cover the existing visible-item codec; client rendering was not observed.

The pre-existing owner update for same-slot entry or Random Property changes made by debug reducers is outside this peer Relay fix.

Made with GPT-6-Astra and GPT-6.1-Sol through the Codex harness in T3 Code. Adversarial reviews used GPT-6-Astra.