[Reactivity] Client patch loop in internal/clientrt

[cssbruno]

Implement the client patch loop for realtime reactivity.

Scope:

• Receive server presentation events.
• Apply bounded DOM updates through compiler-owned client runtime code.
• Avoid user JavaScript ownership of trusted app behavior.

Acceptance criteria:

• Client runtime applies supported patches deterministically.
• Tests cover mount, patch, and cleanup behavior.
• Unsupported patch shapes fail safely.