[stacked] Preserve rail style in generated buffers and reloads

[barrettruth]

Part of #358.

Goal

Persist generated-buffer rail style so stacked buffers remain stacked across refresh and :edit reloads.

Scope

• Store a generated buffer rail style, for example b:diffs_rail_style = 'dual'|'single', alongside existing rail width vars.
• Add a helper to read the buffer rail style with dual as the compatibility default.
• Extend generated buffer creation to accept a rail style and pass it to rail annotation.
• Extend generated buffer replacement/reload to preserve the existing buffer rail style when no explicit override is supplied.
• Ensure generated hunk action metadata is still parsed from raw diff lines, not rail-prefixed display lines.

Non-goals

• Do not introduce a public config option.
• Do not store rail style in source metadata unless the implementation needs it for reliable reloads.
• Do not alter default unified buffers.

Acceptance criteria

• Generated buffers carry rail width, separator width, and style metadata.
• Replacing generated diff lines preserves single rail style when the buffer was created as single rail.
• commands.read_buffer() / :edit reload does not revert stacked buffers back to dual rails.
• Raw hunk metadata remains rail-free and usable for hunk actions.

Verification

• Add command/reload specs proving single rail style survives replacement/reload.
• Add a regression assertion that stored hunk line text remains raw unified diff text.