Port character-level diff highlights to the side-by-side renderer

[Cannon07]

Motivation

The inline renderer in lua/code-preview/diff.lua highlights which characters within a changed line differ between the original and proposed content (extmarks with ClaudeDiffInlineAddedText / ClaudeDiffInlineRemovedText priority 200, layered on top of the line-level highlights). The side-by-side renderer relies on Neovim's built-in :diffthis, which only does line-level highlights — character-level changes are not visually distinguished.

Bringing char-level highlights to side-by-side is the only piece of feature-floor parity worth porting between the two renderers (see ADR-0003). Other inline features either already exist for side-by-side via :diffthis (]c / [c navigation) or don't translate to a two-window layout (the custom statuscolumn with old|new line numbers).

Scope

• Extract char_diff_ranges and the char-extmark application logic from the inline path into something reusable.
• On the side-by-side renderer (show_diff after the layout == "inline" branch), apply matching extmarks to both the CURRENT and PROPOSED buffers for each changed line pair.
• The existing line-level diff colouring from :diffthis stays; char-level highlights layer on top at higher priority.

Out of scope

• New rendering features for side-by-side beyond char-level highlights. After this lands, the side-by-side renderer is feature-floor-complete per ADR-0003 and closed to further additions.
• Statuscolumn changes — old|new line numbers don't apply to a two-window view.
• Inline-specific ]c / [c keymaps — side-by-side already gets these from :diffthis.
• Any change to which renderer is the default layout.

Acceptance

• Side-by-side previews show character-level highlights on changed lines for both CURRENT and PROPOSED windows, matching the inline renderer's behaviour for the same diff.
• Highlight groups are configurable from the same highlights.inline (or a sibling) config — no new duplicate config surface.
• Existing tests pass; a new test covers char-level extmark presence on the side-by-side path.