feat(explorer): Virtual line spacer between Conflict, Staged and Unstaged groups

[kikefdezl]

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Description

When the explorer displays dozens of files, it is easy to lose track of the boundaries between Staged, Unstaged, and Conflict groups, leading to accidental staging or unstaging of the wrong files.

This PR introduces a single-line virtual spacer above group nodes to make it instantly clear where one section ends and another begins.

Implementation Details

This is handled7 at render time.

• A dynamic line_spacers table is generated during Tree:render().
• Any group node that isn't the first visible line in the buffer gets a margin of 1.
• The tree renderer simply loops over this table and applies native virt_lines_above = true extmarks.
• A test was added for this new render-time behavior

Configuration / Opt-in

This PR applies the visual separator universally without exposing new configuration options, as it feels like a sensible and universally beneficial default for the UI. However, if you prefer this to be configurable or opt-in, I would be happy to update the PR to expose a setting for it!

[h0adp0re]

What if only "Staged Changes" is present?

[kikefdezl]

What if only "Staged Changes" is present?

If the Unstaged group visibility is toggled off, the Staged group will stick to the top because we only add the margin to groups that are N > 1 (first always sticks to top).

[kikefdezl]

I made some modifications, the changes are slimmer now.