Align in-memory I/O position controls - #9675

Summary

• Convert BytesIO/StringIO.seek(pos, whence) arguments in order and keep BytesIO positions within the signed-size range, including negative relative seeks and overflow.
• Check closed/exported state before truncate's index conversion using shared typed argument binding; release storage locks during callbacks and recheck state afterward.
• Reject overflowing nonempty BytesIO writes before allocation, while preserving empty writes at the maximum position.

Uses existing numeric conversion and buffer-resize guards, with no per-method FuncArgs parser. Post-index closed/export rechecks are intentional Rust safety checks: CPython's truncate bodies check those conditions before conversion only.

Reference: CPython 3.15.0 BytesIO, StringIO, and seek argument conversion.

Validation

• Bounded CPython 3.15 reference probes confirmed conversion order, closed/exported precedence, negative seeks, overflow, and truncate callbacks that move the position.
• Rust formatting and git diff --check passed.
• Focused host-env, VM and stdlib package check and strict Clippy passed against the final source. Normal repository commit hooks passed.
• RustPython runtime tests, full workspace/C-API tests, and cross-platform execution were not run. No test, Lib, workflow, or expected-failure marker edits.

AI assistance: OpenAI Codex performed source comparison, implementation, reference probes, and independent source review; model version was not exposed.