feat: Consolidate HTML report layout for better UX

- Move stats inline with "Diff Report" title (0 changes | +2 added | Mode: Roslyn Semantic)
- Consolidate file paths to single rows (Old file: path [buttons])
- Add consistent header to top diff section with:
  - Expand/collapse arrow
  - Stats summary
  - JSON and Diff copy buttons (right justified)
- Top diff section now matches individual change section styling

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: randlee