docs(perf): audit profiling methodology for 100k-file workload

[oferchen]

Summary

• Catalogs predicted hot paths in oc-rsync for ~100k-small-file transfers, citing receiver, sender, file-list, and metadata code paths.
• Lists existing optimizations already in tree (parallel stat, parallel dir metadata, sender syscall hygiene) so future perf work doesn't duplicate them.
• Recommends a profiling methodology runnable in the rsync-profile podman container.

Test plan

• Docs-only change; CI fmt/clippy/build matrix should pass without code edits.