chore: pin perf baseline for feat/optimize

Measured `time ./unittest` on -O3 build (best of 3 runs):
  user time = T0 = 114.13s

All subsequent perf commits on this branch quote their best user time
and the cumulative delta vs. this baseline.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: rijnb