chore: add tools/benchmark-speedup.sh

[laulauland]

Reproducible harness for comparing the perf of two drift releases on any drift-enabled repo. Pulls both release tarballs into a temp dir, smokes each, then runs hyperfine.

Usage

From the root of any repo with drift.lock:

# defaults: OLD=v0.8.1 (sequential), NEW=v0.9.1 (all async), 20 runs
tools/benchmark-speedup.sh

# also run the all-sigs-stale variant that exercises parallel blame
tools/benchmark-speedup.sh --stale

# arbitrary versions
OLD=v0.9.0 NEW=v0.9.1 tools/benchmark-speedup.sh

Requires hyperfine on PATH (e.g. brew install hyperfine).

The script temporarily rewrites drift.lock in the --stale mode (replaces every sig with zeros so every anchor reports changed_after_baseline) and restores it at the end via mv.