qkc/slave 04: implement baseConn and XshardConn compatibility layer

[iteyelmp]

This PR introduces the foundational connection layer for cluster communication.

The main goal is to reproduce the Python cluster connection behavior in Go,
while keeping business logic migration out of scope.

Implemented

baseConn

Introduces the common connection abstraction used by future cluster
communication components.

Implemented:

• frame send/receive lifecycle management
• RPC request/response framework
• RPC ID tracking and validation
• pending RPC management
• connection close propagation
• handler dispatch and error handling
• OpSerializer based payload serialization

XshardConn

Implements slave-to-slave communication on top of baseConn.

Implemented:

• 0-byte metadata transport for slave-slave connections
• PING/PONG identity exchange
• peer identity validation
• shard list validation
• xshard transaction request interfaces
• protocol compatibility stubs for unported business handlers

XshardPool

Adds connection management for xshard peers.

Implemented:

• shard-indexed connection tracking
• inbound connection handling
• duplicate slave detection
• connection cleanup and shutdown handling

Full Codex 5.6 review completed: yes

[qzhodl]

Please update the PR description to state explicitly whether a full Codex 5.6 review was run against the complete diff before submission, for example: Full Codex 5.6 review completed: yes/no.

My first Codex review of this PR found multiple correctness and lifecycle issues that the existing tests did not expose, including successful acknowledgements for discarded xshard data, unchecked response error codes, stale multi-shard routes, and timeout/late-response teardown. Please run a complete Codex review before submitting code, address or explicitly document its findings, and disclose the review status and any intentional incomplete stubs in the PR description.