[stacked] Final docs and architecture review

[barrettruth]

Part of #358.

Goal

Perform a final holistic review of stacked layout before closing the tracker, after the implementation, docs, and verification issues have landed.

This is a review/refinement issue, not a feature expansion issue.

Scope

• Re-read the final vimdoc for ++layout=stacked and confirm the public wording is accurate, concise, and consistent with existing help style.
• Specifically review the helpdoc phrasing for:
  • :Gdiff and :Greview signatures and parameter descriptions;
  • side-aware line-number semantics;
  • the ASCII example around replacements;
  • highlight group wording for dual and single rails.
• Review the final code shape across the stacked-layout PR series for small architecture cleanups that are easier to see after the full feature exists.
• Look for unnecessary duplication, awkward naming, over-scoped helpers, missing ownership boundaries, or stale comments/types introduced during the series.
• Confirm the final implementation still matches Tracker: stacked generated diff layout #358’s accepted non-goals: no public config, no split-layout behavior changes, and no rewrite of generated hunk metadata.

Non-goals

• Do not add new stacked-layout behavior unless it is needed to fix a bug found during review.
• Do not rename public user-facing options after docs have landed unless the tracker owner explicitly approves it.
• Do not perform broad unrelated refactors.

Acceptance criteria

• Helpdoc language is reviewed and adjusted if necessary.
• Any small stacked-layout code cleanups are made, or explicitly deemed unnecessary.
• The final code and docs still align with Tracker: stacked generated diff layout #358’s accepted decisions.
• The root tracker can be closed after this issue and the final verification issue are complete.

Verification

• Run docs checks if vimdoc changed.
• Run targeted tests for any code touched by review cleanup.
• If no changes are required, note the review result in the PR body for this issue.