Implement stable entities, ownership, containers, and lifecycle #37

Outcome

Provide the authoritative identity and containment model shared by maps, actors, items, multipart objects, persistence, and protocol projection.

Scope and invariants

• Use non-reused or generational runtime handles plus stable persisted/content identities; define which identity crosses each boundary.
• Centralize create, attach, move, split, merge, detach, and destroy transactions with ownership and lifecycle events.
• Represent inventory/container membership without cycles, duplicate ownership, orphaned children, or use-after-destroy.
• Define multipart identity and teardown so all parts refer to one authority and cannot be independently persisted or rewarded.
• Bound nesting, fan-out, stack size, transaction size, and cleanup work; make every rejection typed.

Acceptance criteria

• Property tests generate random lifecycle operations and continuously validate reachability, ownership uniqueness, generation safety, and container acyclicity.
• Failed multi-object operations roll back completely and publish no success event.
• Persistence and scene projection consume stable snapshots rather than internal pointers.
• Fuzzed handles and stale commands cannot affect a new entity that reused storage.
• Benchmarks cover dense containers, bulk teardown, and multipart entities within explicit budgets.

Dependencies and parallelization

Depends only on architecture #3 and simulation #32. Inventory #49, maps #38, persistence #39, and protocol scene work can build adapters in parallel once identity types freeze.

Independent implementation

New code and tests are MIT-licensed independent work. Preserve public behavior and issue-level design choices, but do not copy or mechanically translate GPL server/Python implementation or tests. Mixed-license content remains external data with its own notices.