qkc/slave 06: add PeerConn, Dispatcher, and peer routing

[iteyelmp]

Summary

This PR implements PeerConn, the communication-layer abstraction for virtual peer-to-peer connections on the slave, corresponding to Python's PeerShardConnection. A PeerConn represents one external cluster peer on one branch. It provides typed outbound commands/RPCs, inbound dispatch through the PeerHandler interface, and the peer-frame routing support required in MasterConn.

This PR is communication-layer only. Shard ownership, registration, creation/destruction, and cascade cleanup are intentionally left to the runtime layer.

Design

• Virtual, socketless connection: A PeerConn owns no TCP socket. All traffic is tunneled through the shared MasterConn via virtualTransport, which stamps outbound frames with the connection's (branch, cluster_peer_id) and receives inbound frames forwarded by MasterConn.
• Connection-level RPC state: Each PeerConn has its own RPC ID namespace and pending-request state, so multiple peer connections sharing one MasterConn remain independent.
• Delegated business handling: Inbound commands and RPC requests are dispatched through the injected PeerHandler; PeerConn itself contains no shard/business logic.
• Decoupled routing: PeerResolver abstracts peer lookup and branch validation, keeping peer ownership and registry management outside MasterConn.

Review Notes

• cluster_peer_id == 0 is rejected at construction. In the Python implementation, 0 is reserved for master-local traffic and is never assigned to a peer connection, but PeerShardConnection does not reject it when constructed; the check only happens when writing a frame. Go validates this invariant at the PeerConn boundary instead, preventing an invalid peer connection from being created in the first place.

• MasterConn → PeerConn cascade cleanup is intentionally not implemented here. PeerConn lifecycle ownership belongs to the runtime layer, which will handle registration, destruction, and cascade cleanup.

• An invalid forwarding branch closes the MasterConn. routeFrame validates the branch against the global configured shard set. A branch outside that set indicates an invalid frame on the master-slave channel, so the entire MasterConn is closed. A globally valid branch with no locally available PeerConn is instead dropped, matching Python's NULL_CONNECTION behavior.

• Python permits writes through a locally closed virtual connection; Go intentionally rejects them. This was investigated and confirmed to be a Python lifecycle/cleanup artifact rather than a protocol requirement, so the behavior is deliberately not reproduced.

Full Codex 5.6 review completed.