Decouple neo-tree concerns from diff.lua (mark_change_and_reveal)

[Cannon07]

Motivation

lua/code-preview/diff.lua contains mark_change_and_reveal, which:

• Sets a status in code-preview.changes registry.
• Calls code-preview.neo_tree.refresh().
• Computes a reveal target (falling back to nearest existing ancestor for created files).
• Shells out to git rev-parse --show-toplevel when reveal_root = "git".
• Calls code-preview.neo_tree.reveal(...) via vim.defer_fn.

The diff renderer should not know any of this. Today, modifying neo-tree reveal behaviour means editing diff.lua; testing the reveal logic in isolation is impossible because it's invoked as a side effect of show_diff.

Proposed direction

Move the neo-tree coordination into either:

• lua/code-preview/neo_tree.lua — expose a single high-level entrypoint like neo_tree.mark_and_reveal(file_path, action, cfg) that handles status mapping + refresh + reveal target computation internally; or
• A new thin coordinator module (lua/code-preview/coordinator.lua or similar) — if neo-tree should stay agnostic about when it's called.

diff.show_diff then just calls that one entrypoint and stops knowing about reveal targets, git roots, or vim.defer_fn timing.

Scope

• Pure refactor. No behaviour change visible to the user.
• Touches lua/code-preview/diff.lua, lua/code-preview/neo_tree.lua, and possibly a new module.
• Add a unit test for the reveal-target fallback logic (the created → nearest-existing-ancestor walk).

Out of scope

• Changing the reveal behaviour itself (defer-300ms, rm-wins precedence, git/cwd choice).
• The wider diff.lua split.