[stacked] Final verification for generated diff actions and shim

[barrettruth]

Part of #358.

Goal

Perform the final end-to-end verification pass for stacked generated diffs before closing the tracker.

Scope

• Run targeted automated specs added throughout the stacked layout work.
• Run broader project checks appropriate for the final implementation.
• Smoke-test the real-repo shim at /tmp/diffs/gdiff-stack-shim-today with the candidate worktree on runtimepath.
• Verify generated diff actions and navigation still work with stacked rails.

Required shim coverage

At minimum exercise:

• worktree.lua: :Gdiff ++layout=stacked, hunk navigation, <CR> source open, dp, and Visual dp.
• staged.lua: staged generated diff equivalent where supported, hunk navigation, do, and Visual do.
• mixed.lua: ensure stacked layout does not change staged/unstaged edge selection.
• untracked.lua: all-added stacked generated diff and dp staging.
• conflict.txt: :Gdiff ++layout=stacked generated merge diff and conflict maps.
• :edit reload on a stacked generated buffer preserves single rails.

Non-goals

• Do not add new product behavior in this verification issue.
• Do not relax existing tests to make stacked pass.

Acceptance criteria

• Targeted specs pass.
• Relevant broad checks pass.
• Shim smoke confirms hunk navigation, source open, hunk actions, visual hunk actions, untracked handling, and conflict helpers with stacked rails.
• Any regressions are filed or fixed before Tracker: stacked generated diff layout #358 is closed.

Verification

Use the project workflow from AGENTS.md, including direnv exec . for commands and the shim runtimepath pattern documented there.