fix(brainfs): check every component of the walk root (#157)

Review round 2 on PR #166: Lstat on the walk root checks only its last
component, so search with Wing+Hall (root wiki/<wing>/<hall>) walked a
symlinked <wing> whose target had a real <hall> directory.

WalkRegular now Lstats every prefix of the root and requires each to be
a real directory. New brainfs and search (Wing+Hall) tests; the Wing-only
search test is renamed to say what it covers.

The doc now records the accepted residual: a directory swapped for a
symlink between listing and read is still followed; os.Root keeps the
read inside the brain but not inside the walk root.

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

ingestion/internal/brainfs/brainfs_test.go

@@ -172,3 +172,20 @@ func TestWalkRegularReportsASymlinkedWalkRootAndYieldsNothing(t *testing.T) {
 	require.Len(t, errs, 1)
 	assert.ErrorIs(t, errs[0], ErrNotRegular)
 }
+
+func TestWalkRegularReportsASymlinkedIntermediateComponentOfTheWalkRoot(t *testing.T) {
+	dir := t.TempDir()
+	require.NoError(t, os.MkdirAll(filepath.Join(dir, "real", "sub"), 0o755))
+	require.NoError(t, os.WriteFile(filepath.Join(dir, "real", "sub", "n.md"), []byte("n"), 0o644))
+	require.NoError(t, os.Symlink("real", filepath.Join(dir, "alias")))
+	b, err := Open(dir)
+	require.NoError(t, err)
+	defer func() { _ = b.Close() }()
+	var got []string
+	var errs []error
+	require.NoError(t, b.WalkRegular("alias/sub", func(rel string) error { got = append(got, rel); return nil },
+		func(_ string, err error) { errs = append(errs, err) }))
+	assert.Empty(t, got)
+	require.Len(t, errs, 1)
+	assert.ErrorIs(t, errs[0], ErrNotRegular)
+}