Split lua/code-preview/diff.lua into focused modules

[Cannon07]

Motivation

lua/code-preview/diff.lua is 599 lines covering four loosely related concerns:

1. The side-by-side renderer (show_diff after the layout == "inline" branch).
2. The inline renderer (show_inline_diff, build_inline_diff, char_diff_ranges, inline_statuscolumn, apply_inline_highlights).
3. The active-previews registry (active_diffs, is_open, close_for_file, close_diff_and_clear, _active_diffs).
4. Cross-cutting neo-tree coordination (mark_change_and_reveal) — already tracked separately.

This mixing makes the file hard to extend and impossible to test in pieces. Per ADR-0003, the inline renderer is the strategic direction, so the seam between renderers is also a natural extension point.

Proposed direction

Extract along these lines (final names up for discussion):

• lua/code-preview/diff/init.lua — public entry (show_diff, close_for_file, etc.). Owns the active-previews registry.
• lua/code-preview/diff/inline.lua — inline renderer (build, highlight, statuscolumn, keymaps).
• lua/code-preview/diff/side_by_side.lua — :diffthis-based renderer.
• lua/code-preview/diff/util.lua — shared helpers (read_file_lines, char_diff_ranges if both renderers end up using it after Port character-level diff highlights to the side-by-side renderer #56).

Renderers expose render(...) → {tab, bufs, augroup, inline_win?}; the registry doesn't care which one ran.

Scope

• Pure refactor. No behaviour change. No public API change (require('code-preview.diff').show_diff etc. continue to work).
• Existing tests (tests/plugin/diff_lifecycle_spec.lua) must pass unchanged.

Out of scope

• Path-parameter rename (Refactor: clean up path parameter naming in diff.show_diff #55) — should land before this so the new modules use clean names from day one.
• Char-level highlights port (Port character-level diff highlights to the side-by-side renderer #56) — easier after the split.
• Decoupling mark_change_and_reveal from diff.lua — tracked separately.
• Changing the default layout or removing the side-by-side renderer.

Dependencies / ordering

Sequence with the rest of the refactor backlog:

1. Refactor: clean up path parameter naming in diff.show_diff #55 (path rename) — clean names first.
2. This split.
3. Port character-level diff highlights to the side-by-side renderer #56 (char highlights for side-by-side) — easier to land after the inline renderer is its own module.
4. The neo-tree decoupling issue.