Add ASGI scope access-log atoms

[Gonghan-Princess]

Summary

Add a non-breaking {Variable}s access-log atom for string and byte values in the request's ASGI scope, as requested in #359. This makes request-specific trace IDs loggable without storing them in process-wide environment variables. The existing {Variable}e atom remains unchanged.

Scope byte values use Latin-1, consistent with the existing header atoms. Missing keys and unsupported value types render as -. Documentation and a regression test cover formatting, bytes, unsupported values, and coexistence with environment atoms.

Verification

• pytest tests -q with seven known Windows/incompatible cases deselected: 190 passed, 7 deselected. Without deselection, the six failures are identical on the pristine base checkout (186 passed there, 187 passed with this change); the seventh case uses Unix /tmp/path for a file handler.
• Focused logging tests: 11 passed, 4 deselected.
• Black, isort, flake8, git diff --check: pass.
• Mypy on changed Python files with --no-warn-unused-ignores: pass. Unmodified base already has obsolete type: ignore and Windows-only socket typing errors with the installed mypy version.
• Sphinx documentation build with -W --keep-going after sphinx-apidoc: pass.
• Source distribution and wheel build: pass.

This contribution was prepared with AI assistance and verified against the pristine base checkout.