fix: make fork-at-point reliable in deep/large sessions

[dnouri]

Summary

• fork-at-point now resolves entries via get_fork_messages.
• mapping keeps visible-turn behavior in compacted sessions (last-N alignment).
• explicit fork-at-point RPC error message for failed get_fork_messages responses.
• RPC response dispatch now handles id-less responses by:
  1. matching pending request command type, then
  2. falling back to the sole pending request.

Tests

• fork-at-point mapping tests: normal, compaction, deep history, before-first-heading.
• fork-at-point explicit RPC failure message test.
• core id-less response routing tests (command-match and sole-pending fallback).
• process-exit cleanup test for pending command metadata.

Validation

• make test ✅
• make check ✅