[stacked] Wire stacked layout through Gdiff and Greview

[barrettruth]

Part of #358.

Goal

Wire ++layout=stacked through :Gdiff and :Greview as generated single-column buffers with single side-aware rails.

Scope

• Map generated layouts to rail styles:
  • unified -> dual;
  • stacked -> single.
• Keep split routed to the existing paired endpoint/native-diff behavior.
• Make :Gdiff ++layout=stacked open a generated diffs:// buffer, not split endpoint windows.
• Make :Greview ++layout=stacked open the full generated review map, not the two-surface split workspace.
• Pass single rail style through special generated paths such as unmerged direct :Gdiff handling.
• Preserve :Gvdiff/:Ghdiff behavior: parser support can work, but completion remains object-focused unless intentionally changed.

Non-goals

• Do not change the default layout.
• Do not change split workspace behavior.
• Do not add config.

Acceptance criteria

• :Gdiff ++layout=stacked opens one generated diff buffer with single rails.
• :Greview ++layout=stacked opens one generated review buffer with single rails and normal quickfix/location-list behavior.
• ++layout=split behavior remains unchanged.
• Unmerged generated diff paths preserve stacked rails when invoked through stacked layout.

Verification

• Add command behavior specs for Gdiff and Greview stacked layout.
• Add assertions that stacked layout opens generated buffers rather than split endpoint windows.
• Run existing split-layout specs to guard against regressions.