Deduplicate request dispatch logic in pipe and TCP handlers

[mark-e-deyoung]

Problem

handle_client (pipe) and handle_socket_client (TCP) share ~80% of their logic: snapshot capture, LRU eviction, session management, event polling, pin/unpin, and the large if/else-if dispatch chain. Any bug fix to one must be replicated in the other.

Proposed Solution

Extract a shared function that both handlers call, leaving only the transport-specific read/write in each handler.

References

• Correctness audit item 10
• daemon/src/server.cpp:51-257
• daemon/src/tcp_server.cpp:90-343