Feature idea: side-by-side (split) diff view (builds on #2997)

## Summary

An opt-in **side-by-side (split)** diff view for the diff component, toggled with `Shift+V`, alongside the existing unified view. It's **view-only** and **defaults to unified**, so nothing changes unless you turn it on. It lives in the shared diff component, so all diff hosts (status, compare-commits, inspect-commit, file-history) get it with **no host changes**.

This **builds on #2997** (syntax highlighting in the diff view): it reuses that PR's per-line highlight cache and change-status styling. So it's naturally a follow-up rather than a standalone change, and I wanted to check whether you'd welcome it before opening a PR.

## Motivation

Unified diffs interleave `-`/`+` lines, which can be hard to scan for edits and reflows. A side-by-side view puts the old text on the left and the new on the right, aligned, which many people find easier to read at a glance.

## What it adds

- **`Shift+V`** toggles split ⟷ unified (persisted per-repo; default unified). The command hint shows the next state ("Split view" / "Unified view").
- **View-only:** staging and hunk-jumping stay unified-only; movement, copy, edit, and syntax highlighting work in both. Deletions align on the left, additions on the right (paired by position); context and hunk headers span both columns.
- **Reuses existing data:** a small pure `build_split_rows(&FileDiff)` state machine over the existing `FileDiff`; the syntax-highlight cache from #2997 maps for free (left cell → old side, right → new). **No new dependencies.**
- Narrow-terminal fallback (a centered hint below a minimum column width); binary/empty diffs fall back to unified.

## Dependency on #2997 (heads-up)

Split reuses the syntax-highlight machinery from #2997 (`diff_highlight` cache, `DiffHighlightStyle` tint, a shared span-clipping helper), so it applies cleanly on top of #2997 but not on `master` alone. If you're open to the feature, the natural options are: (a) I open it as a follow-up PR once #2997 merges, (b) I stack it on #2997, or (c) fold it into #2997 — whichever you prefer.

## Status

Implemented on my fork; `make check` is green (fmt + clippy `--all-features` + nextest + tombi + deny; 343 tests, incl. 12 new for the row-alignment model). Defaulting to unified keeps the snapshot tests unchanged. Happy to open the PR in whatever form suits you.

まず #2997 と共有の diff コンポーネントを確認し、特に言及されている diff_highlight キャッシュ、DiffHighlightStyle の tint、span-clipping ヘルパーを確認してください。split view が引き続き opt-in で、デフォルトが unified であること、unified-only staging と hunk-jumping が維持されること、狭い端末・バイナリ diff・空の diff を処理できること、そして row-alignment tests を含めて make check に通ることを確認してください。

索引モデルが issue の本文から書いたものです。