Preview uncommitted changes in the detail pane

For a dirty submodule, list the changed files so the user can remember
what they were doing there:

  Changes   5 files · 2M 1A 1D 1R · +37 −12
            ──────────────────────────────────
            M  render.go  src/render/  +20 −4  3d ago
            ?  scratch.txt                     5m ago

- Status.Changes: files changed against HEAD (staged and unstaged
  combined) with kind, line counts and modification time; sorted by kind,
  then by size; capped at 200 with totals over all files.
- Git provider: parse `status --porcelain=v2 -z` and one
  `diff HEAD --numstat -z -M`, only for dirty submodules.
- Detail pane: flat list with a colored status letter per file, like IDE
  source-control views; counts and age in compact columns; up to 10 files,
  every kind keeps a line, the rest is summarized as "n more".
- Fixture gains libs/work with every kind of change; mock data extended.
- Docs: F24, change preview in the UX guide, D17.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/requirements.md

@@ -61,6 +61,13 @@ commit hashes. It doesn't answer the questions users actually have:
 - **F22** Lists the commits between pin and tip (SHA, subject, author, age)
   with a title stating ahead/behind/diverged.
 - **F23** Shows child count, issues below, and the remote URL.
+- **F24** For a submodule with uncommitted changes, previews the changed
+  files so the user can remember what they were doing: a flat list with a
+  colored letter per file (`M` modified, `A` added, `D` removed, `R`
+  renamed, `?` untracked, `!` conflict), line counts (`+n −m`) per file and
+  in total, and how long ago each file was edited. Only a handful of files are
+  shown; the rest is summarized as "n more" with their kinds. Staged and
+  unstaged changes are combined.
 
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