fix(ui): make word wrap work in side-by-side diff view

[yuhua99]

Fixes #441.

Sorry for the delay on this one — work has been busy the past while, so
it took me a bit to get the PR out.

Fix

Introduce a span-level word wrapper (wrap_spans) and route both views through it, so the same code that renders rows also produces their heights. Side-by-side additionally wraps each column independently and keeps the │ divider aligned on continuation rows.

Commits

1. feat(ui): add span-level word wrapper wrap_spans:
   style-preserving word wrapper primitive; no callers yet.
2. refactor(ui): share wrap-aware cursor-line highlight across diff views:
   extract a common paint_cursor_line_highlight that walks row_heights so unified and side-by-side can't drift on wrapped rows.
3. refactor(ui): simplify side-by-side pair loop and expanded-context helper:
   some cleanup to make room for the meta insertion in handle multiple sessions / multiple repositories #5.
4. fix(ui): single-pass wrap in unified view so row heights match rendered rows:
   replace Paragraph::wrap + compute_row_heights dual pass with one wrap_spans call producing both the visual rows and their heights.
5. feat(ui): per-column word wrap in side-by-side view:
   the actual fix. Record SbsRowMeta (raw content, prefix, pad style) per logical line at build time, then wrap each column at content_width with │-preserving continuation prefixes. Deletes the now-unused compute_row_heights.
6. fix(ui): extend comment-box gutter bar over wrapped rows:
   draw the comment-box vertical bar across every wrapped sub-row instead of stopping at the first visual row.
7. feat(ui): replace front-draining with cursor walk in wrap_spans hard split:
   perf. make it faster

[agavra]

Thanks @yuhua99 this is a clear improvement. Some things don't work great (like zz doesn't always compute the correct center) though we can file those as follow ups as we figure it out.