perf(runner): reuse unchanged file metadata

[Chemaclass]

🤔 Background

Related #1372

Header counting discards metadata needed again during execution. Reuse unchanged short-file scans while checking complete contents before replay, so earlier files can still rewrite later ones.

💡 Changes

• Reuse provider and annotation metadata for unchanged files up to 512 characters in sequential multi-file runs with a header.
• Preserve discovery and dynamic-file counting, with a fresh scan whenever content changes or exceeds the bound.
• Cover same-size rewrites, scan-time changes, reordered paths, scan budgets and total-count parity. Update the changelog.
• Source, standalone, static and formatting gates pass. Two 40-file fixtures improve by 9% and 7%, including freshness checks. Medians use nine alternating pairs against main at 5731afa on macOS Bash 3.2 with the same paths and fixtures; repeated larger-file medians differ by less than 1%.