The first capture the repository can attribute to a commit

Benchmark artifacts begin recording engine commit, dirty flag, competitor hashes and per-repetition schedules, after a harness was found able to name a commit it had never measured.

f1b38086: "bench: the first capture this repo can attribute to a commit". The sampling profiler the roadmap had wanted since its third ticket landed the day before. The ledger's measurement-hygiene section records the failures that motivated this: a profiler "lying about a quarter of json-large", a harness that "could name a commit it had never measured", and a leftover ZIPP_GC_STRESS=1 process from a killed session that held a core for hours and was misdiagnosed as an engine livelock.