Add isolated decode_message benchmark for measuring copy-reduction impact

Standalone benchmark (no cluster required) that constructs synthetic
RESULT/ROWS wire-format bodies and measures ProtocolHandler.decode_message()
throughput across 8 scenarios (small to 16MB, narrow to 20-column wide).

Pins to a single CPU core via sched_setaffinity for consistent results.
Supports both Cython and pure-Python paths, with --cprofile option.

Author: mykaul