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/decisions.md

@@ -128,3 +128,21 @@ processes.
 **Decision.** The git provider limits concurrent `Status` calls to
 `runtime.NumCPU()` with a semaphore that respects `ctx`.
 **Consequences.** The UI stays unchanged; rows fill in steadily.
+
+## D17 — Change preview: flat letter list against HEAD
+
+**Context.** `changed` doesn't tell users what they changed or why; they
+want to remember what they were doing in a submodule.
+**Decision.** `Status.Changes` lists the changed files against HEAD (staged
+and unstaged combined) with line counts. The detail pane shows them as a
+flat list with a colored status letter in front, like IDE source-control
+views, instead of group headers. Sorted by kind, then by size; the 10-line
+budget keeps one line for every kind present and summarizes the rest.
+Placed directly below the issue lines, because the pane is cut by height.
+One summary line for all files, then strictly one line per file with its
+own age: a second summary line ("+1 · edited 19m ago") read as if it
+belonged to the first file.
+Dirt inside nested submodules is ignored (D7); unstaged moves show as
+removed + untracked.
+**Consequences.** One extra `git diff` per dirty submodule, none for clean
+ones. Not included: staged/unstaged markers, diff bodies, stash count.