GeezerVison

Summary

Keep mmap cursor reads/updates under the mapping guard and preserve the byte-string result of sequence iteration. Includes the whole canonical mmap free-threading test file.

This is a functionality-based extraction from #9439, based on main 84d7afc2d59e05c66c92df3ce0854679d2b32b52. The original aggregate is being narrowed to monitoring. The extracted files are preserved exactly from the reviewed aggregate source.

All included Lib files were regenerated with actual scripts/update_lib migrate from CPython d220409a13b77de7a6c30abe16ea90db01528edc and match that pin byte-for-byte. No test file is split across PRs and no assertions or skips are added.

Validation and limits

Earlier aggregate: 30 bounded serial anonymous-mmap checks are historical evidence from a different combined executable. No new local mmap concurrency, resize/flush race or crash test was run.

Fresh standalone cargo check -p rustpython-stdlib --lib --no-default-features --features compiler,threading,host_env,doc,ucd_3_2_0 and matching strict Clippy (-D warnings) passed on the 84d7 base, with locked/offline dependencies and source hashes verified before and after. No runtime test was run for this extraction. Exact-head hosted CI: pending. Old aggregate green checks are historical, not standalone validation of this extraction.