Benchmark managed Python dylib patching on macOS (#22216)
## Summary
Adds benchmarks for our current managed Python install baseline on
macOS, i.e. dylib patching with `install_name_tool`.
The main benchmark installs from a cached CPython download into a fresh
directory with `--offline`, measuring the full install without download
time. A separate benchmark isolates dylib patching.
## Test Plan
Benchmark changes only. Verified the warm install in Criterion test
mode; Clippy and formatting pass.
