Fix TSAN warnings

[Akilan-Sivakumar]

• This PR fixes data races flagged by ThreadSanitizer (TSAN) across the encoder's multi-threaded code paths, including encoder shared state, TME, rate control row stats, and weight analysis.
• It also includes code-quality fixes addressing issues reported by the PR CI pipeline.
• TSAN tested across all presets (8-bit and 10-bit) and across all CLIs in the x265 regression suite. Smoke, regression, performance, and quality tested.