Avoid C414 for stable multi-key sorted calls

[onuracar-dev]

Fixes #661.

Summary

• Do not emit C414 for sorted(sorted(...), key=<non-None>): Python's stable sort allows the inner sort to establish a secondary ordering.
• Conservatively avoid C414 when an outer ** expansion may provide the key.
• Preserve C414 for key=None and the existing genuinely redundant cases.
• Document the exception in the C414 reference and unreleased changelog.

Validation

• Upstream regression: the new explicit-key case failed with the reported C414 diagnostic before the fix.
• Expansion regressions: the **kwargs and literal **{"key": len} cases also failed before their guard was added.
• Focused C414 tests: 25 passed, 126 deselected.
• Full suite with ResourceWarning, DeprecationWarning, and PendingDeprecationWarning promoted to errors: 151 passed.
• pre-commit run --all-files: all hooks passed, including RST checks, Ruff, formatting, and MyPy.
• uv build: wheel and sdist built successfully.
• git diff --check and staged secret scan: passed.

AI disclosure: OpenAI Codex (GPT-5) analyzed the issue and generated/revised the implementation, tests, documentation, changelog text, commit message, and this pull-request description. Independent Codex review passes found and corrected a remaining **kwargs false positive before publication, then re-reviewed the final commit and validation results. No separate human review is being represented here.