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/architecture.md

@@ -15,16 +15,16 @@ internal/submodule/           domain model; NO UI imports
   provider.go                 Provider interface
 internal/submodule/git/       read-only provider using the git CLI
   exec.go                     git runner (environment, errors)
-  parse.go                    pure parsers for git output
+  parse.go                    pure parsers for git output (status, numstat, …)
   provider.go                 Tree, Status, pin → ref resolution
 internal/submodule/mock/      fixture provider with artificial latency
 internal/tui/                 Bubble Tea application
   app.go                      root model: layout, message routing, loading
   keys.go                     key bindings (also feed the help view)
   msgs.go                     treeLoadedMsg, statusLoadedMsg
-  theme/                      colors, glyphs, issue words, level styles
+  theme/                      colors, glyphs, issue words, change letters, level styles
   tree/                       tree component: rows, navigation, mouse, view
-  detail/                     detail pane
+  detail/                     detail pane; changes.go: change preview
   header/                     header line
   footer/                     help and flash messages
 ```