feat(diff): syntax-highlight the unified diff view (async, cycling styles)

Each diff line's foreground is now syntect-highlighted (reusing the
in-tree two-face syntax set that already powers the file/blame viewer
— no new deps, no external bat), keeping both added and removed sides.
Change status is shown by a selectable indicator; [Shift+X] cycles
off / gutter (colored bar) / sign (green + / red -) / tint (faint row
background). Default tint. Persisted per-repo.

Highlighting runs off the UI thread on the shared rayon threadpool
(AsyncDiffHighlightJob, modeled on the blame view's AsyncSyntaxJob):
the diff renders plain instantly and colors swap in when ready, so
file navigation never blocks regardless of diff size. The result is
self-polled in draw() and applied only if its diff hash still matches
(stale results discarded). Capped at 10k lines; horizontal scroll
preserved via byte-window clipping; falls back to the previous
red/green rendering when off, over cap, binary, or highlight fails.

Squash of feat/diff-highlight (baa41a2 + e136a64).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BMQxBdr5f3B3jmDtdvGtTu

Author: stevenpollack

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 * support x509 commit signing [[@kaden-l-nelson](https://github.com/kaden-l-nelson)] ([#2514](https://github.com/gitui-org/gitui/issues/2514))
+* syntax highlighting in the diff view ([Shift+X] cycles style: off / gutter / +- sign / tint) [[@stevenpollack](https://github.com/stevenpollack)]
 
 ### Changed
 * use [tombi](https://github.com/tombi-toml/tombi) for all toml file formatting