feat: add a wire-codec endpoint for network transports

[joshua-temple]

Adds the wire-codec seam that lets a network transport carry actor operations between nodes while the kernel keeps its concrete, typed events. System.DeliverWire decodes a JSON-encoded event into the node's own event type E and delivers it locally; SpawnWire decodes a JSON input map and spawns locally. The new WireEndpoint interface (which *System satisfies) is the network counterpart to the in-process Endpoint: where the in-memory transport passes Go values between same-process systems unchanged, a network transport serializes the payload on the sending node and hands the bytes to the owning node's WireEndpoint, which decodes them into E. Decoding on the owning node is what keeps the transport type-erased while the kernel stays typed. This is the stdlib-only foundation for the gRPC transport, which lands in its own module so the cluster core carries no network dependency. Coverage: cluster 93 percent. Additive; stdlib-only.