Define and implement multiplayer-safe stable identity across ECS collision and remint paths

## Problem

Current ECS flows sometimes rely on collision, remint, and copy semantics. A remote client must generally be able to assume that an ID it minted remains stable; reminting after synchronization adds another round trip and can redirect references. Existing endpoint-remap fixes and the remint-boundary ADR do not implement the end-to-end remote-client contract.

## Goal

Establish stable identity semantics that work for local compatibility flows and future multiplayer synchronization.

## Proposed Solution

Define which boundary may remint, how requested-to-final IDs propagate to every reference and peer, which identities must be UUIDs, and how legacy copy/repair behavior is isolated. Implement the agreed contract with serialization and multi-client tests.

## Acceptance Criteria

- The remote-client identity contract is documented and approved.
- Every collision/remint entry point is inventoried and assigned one policy.
- Requested-to-final IDs propagate atomically to all serialized references.
- Multi-client tests cover concurrent creation, collision, copy/import, and reconnect.
- Legacy repair behavior has an explicit compatibility boundary and sunset.