Align deque comparison and insertion controls - #9678

Summary

• Check deque mutation after each count comparison, before invoking any later comparator.
• Reuse index search for containment while preserving successful-match, callback-exception and finalizer ordering.
• Bind insertion indexes through the existing signed machine-width converter and remove the unused sequence-trait implementation.

Comparison references are released outside storage locks. Existing index range handling and remove behavior are preserved.

Reference: CPython 3.15.0 deque count, containment, index and insert.

Validation

• Independent source review, formatting and whitespace checks passed.
• Two independent runs reproduced 80 CPython 3.15 callback cases, with additional index-boundary, identity-shortcut, bounded-deque and comparison-result boolean-callback probes.
• Fresh focused VM, host-env and stdlib check and strict Clippy with rustpython-stdlib/threading passed. Normal repository commit hooks passed.
• RustPython runtime, full workspace/C-API suites, and cross-platform execution were not run. No test, Lib, workflow, or marker edits.

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