Fetch in the background and follow changes in the repositories

- On startup, after the local state is shown, fetch the superproject and
  every initialized submodule (all remotes, prune, at most 8 at once).
  Fetches never prompt; failures show git's message. --no-fetch skips it,
  f fetches again.
- The header shows "fetching n/m", then the fetch age and failures; the
  detail pane shows each submodule's last fetch and fetch errors.
- The git provider polls each repository's metadata (refs, HEAD, index,
  .gitmodules) every 2 s and reports changes. The app reloads only what
  changed, keeping old values, expansion and cursor; loads are
  single-flight with coalesced reruns. Refresh is driven by the
  repositories, so a fetch in another shell updates the view like ours.
- r reloads everything (worktree edits are not watched).
- Docs: F50-F53, N1/N8 rewritten, D18-D20 (D12 superseded).

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

internal/submodule/git/provider_test.go

@@ -27,11 +27,8 @@ func TestMain(m *testing.M) {
 		}
 		defer os.RemoveAll(tmp)
 		if _, err := exec.LookPath("git"); err == nil {
-			_, file, _, _ := runtime.Caller(0)
-			script := filepath.Join(filepath.Dir(file), "..", "..", "..", "scripts", "make-fixture.sh")
-			out, err := exec.Command("bash", script, filepath.Join(tmp, "fx")).CombinedOutput()
-			if err != nil {
-				panic("make-fixture.sh: " + err.Error() + "\n" + string(out))
+			if err := makeFixture(filepath.Join(tmp, "fx")); err != nil {
+				panic(err)
 			}
 			fixtureDir = filepath.Join(tmp, "fx", "super")
 		}
@@ -40,6 +37,16 @@ func TestMain(m *testing.M) {
 	os.Exit(code)
 }
 
+// makeFixture runs scripts/make-fixture.sh; the superproject is dir/super.
+func makeFixture(dir string) error {
+	_, file, _, _ := runtime.Caller(0)
+	script := filepath.Join(filepath.Dir(file), "..", "..", "..", "scripts", "make-fixture.sh")
+	if out, err := exec.Command("bash", script, dir).CombinedOutput(); err != nil {
+		return fmt.Errorf("make-fixture.sh: %v\n%s", err, out)
+	}
+	return nil
+}
+
 func loadFixture(t *testing.T) (*Provider, *submodule.Repo) {
 	t.Helper()
 	if fixtureDir == "" {