Floating diagram mode: implement variable-width reflow and scroll-safe rendering

Split from #3.\n\nImplement the rendering behavior for floating diagrams where chat text reflows around a right-anchored diagram.\n\n## Scope\n- support per-line wrap widths for rows overlapping the floating diagram\n- render text above/below at full width\n- keep left-side-only reflow beside the diagram\n- make scroll/caching logic stable and test-covered\n\n## Success criteria\n- diagram floats on the right and remains vertically centered in viewport\n- rows overlapping the diagram use narrower wrap width on the left side only\n- rows outside overlap use full width\n- scrolling does not produce unstable line counts or visual glitches\n- cache invalidation includes scroll/diagram-relative state where needed\n- visual or automated tests cover representative overlap/scroll cases\n\nBlocked by: #33\nParent: #3

The issue names no files, tests, or entry points; first review the rendering, scrolling, and caching paths after resolving the dependency on #33. Done means representative overlap and scroll cases are covered, with right-anchored vertical centering, left-only narrowed wrapping during overlap, full-width text elsewhere, and stable scrolling without visual glitches.