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/ux-guide.md

@@ -61,12 +61,58 @@ indent ▸ name [n]   status          ref             delta
 2. One line per issue: word (colored, 10 columns) + explanation;
    `in sync  nothing to do` if there are none; `staged` and
    `detached HEAD on pin` as information
-3. `Ref` with distance and `via <rule>`
-4. `Pinned`, `Checkout`, `Tip`: SHA + subject; the checkout SHA is yellow if it
+3. `Changes` (only with uncommitted changes), see
+   [Change preview](#change-preview)
+4. `Ref` with distance and `via <rule>`
+5. `Pinned`, `Checkout`, `Tip`: SHA + subject; the checkout SHA is yellow if it
    differs from the pin
-5. "Pin is n commits behind/ahead of …" / "Pin and … have diverged": commit list
-6. `Children` (count and issues below), `URL`
+6. "Pin is n commits behind/ahead of …" / "Pin and … have diverged": commit list
+7. `Children` (count and issues below), `URL`
 
+#### Change preview
+
+```
+Changes   5 files · 2M 1A 1D 1R · +37 −12
+          ──────────────────────────────────────
+          M  render.go  src/render/  +20 −4  3d ago
+          M  README.md                +3 −1  20m ago
+          A  notes.md  docs/           +14  2h ago
+          D  old.go  src/               −8
+          R  new.go  ← old.go                1d ago
+          ?  scratch.txt                     5m ago
+          …  3 more (2M 1?)
+```
+
+- A flat list, no group headers: IDEs (VS Code, JetBrains) do the same, and
+  headers would use up the few lines the preview has. The letter is the
+  meaning, the color only supports it (N4).
+- Letters and colors: `!` red bold, `M` yellow, `A` green, `D` red, `R`
+  cyan, `?` muted. Line counts: green `+n`, red `−n`; `bin` for binary
+  files, `submodule` for a moved nested pin, nothing for untracked files.
+- Order: kind (`!`, `M`, `A`, `D`, `R`, `?`), then changed lines, largest
+  first, because the biggest edits are the ones people remember.
+- Budget: 10 files. Every kind that occurs gets at least one line, the
+  rest is filled in sort order. `… n more (<kinds>)` summarizes the hidden
+  files. With 11 files all are shown: "1 more" would take the same line as
+  the file itself.
+- The `Changes` line summarizes all files: count, files per kind, line
+  totals. Everything below it describes one file per line, so no line can
+  be mistaken for a file's detail. In a narrow pane the line totals are
+  dropped rather than cut.
+- A muted rule (`─`, `-` in ASCII mode) separates the summary from the
+  files. It starts in the value column and is as wide as the summary or
+  the file table, whichever is wider.
+- The age (muted) is the file's modification time, so the most recently
+  touched files stand out. Removed files have no age.
+- The list starts in the value column (under `5 files`), like every other
+  value in the pane.
+- File name bold, directory muted. Counts form a right-aligned column
+  directly after the widest path (max 48 columns), not at the pane's edge,
+  so they stay next to their file in wide panes. If space is short the age
+  column goes first, then the directory is cut from the left
+  (`…ernal/client/`), the name last. For
+  renames the muted part is `← old/path`.
+- Untracked directories are shown as one entry (`tmp/`).
 ### Header
 
 `path  branch ↑↓ · n submodules · <count> <issue>… · n pin staged · n loading · fetched <age>`