feature: add network object registry and handshake

[PyXiion]

Parent: #13

Create the internal registry and lifecycle model shared by net.shared and net.channel.

Required behavior:

• Server-owned network objects are registered by ID and kind.
• Client lookup returns a pending handle rather than failing.
• Objects can be bound to one player or all players.
• Initial synchronization announces readiness.
• Destruction and player disconnect close handles.
• Duplicate IDs have deterministic behavior.
• Pending operations are woken on readiness or closure.

Acceptance criteria:

• Handles transition through pending, ready, and closed.
• Ready callbacks run exactly once.
• Player-bound objects cannot be observed by other clients.
• Disconnect wakes pending operations.