docs(fast_io): evaluate MAP_POPULATE for mmap'd basis files (#1663)

[oferchen]

Summary

• Audit doc evaluating whether oc-rsync should add MAP_POPULATE to read-only basis-file mmaps
• Covers per-page minor faults vs eager population trade-off, kernel ABI, fallback when MAP_POPULATE unsupported, and interaction with MADV_WILLNEED and io_uring registered buffers
• Recommendation block at end identifies follow-up benchmarks and call sites to instrument

Test plan

• No code change; cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features --no-deps -- -D warnings (no new warnings)