test: pin CreateRegular directly and promote/reject with in-brain targets (#188)

Review round 2 on PR #189: CreateRegular (promote's and reject's only
write path) had no test; replacing mkdirReal with root.MkdirAll, dropping
cleanRel or calling PutRegular all stayed green, and the promote/reject
test planted only outside targets, which os.Root refuses on its own.

- TestCreateRegularCreatesOnlyNewFilesThroughRealDirectories: in-brain
  symlinked parent, unclean path, existing file and dangling link
  (fs.ErrExist, nothing changed), positive control with missing parents.
- TestPromoteAndRejectRefuseAnInBrainSymlinkedDestinationDirectory.

All three flips are red now.

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

ingestion/internal/brainfs/brainfs_test.go

@@ -493,3 +493,39 @@ func TestRemoveRefusesAPathBehindASymlinkedDirectoryAndRemovesOnlyTheLink(t *tes
 	_, err = os.Stat(filepath.Join(dir, "wiki", "n.md"))
 	assert.True(t, os.IsNotExist(err))
 }
+
+// CreateRegular is the only write path for promote and reject: it creates
+// missing parents through the root, refuses an in-brain symlinked parent and
+// an unclean path, and never replaces anything already there (fs.ErrExist,
+// content unchanged), including a dangling link.
+func TestCreateRegularCreatesOnlyNewFilesThroughRealDirectories(t *testing.T) {
+	dir := t.TempDir()
+	require.NoError(t, os.MkdirAll(filepath.Join(dir, "raw", "keep"), 0o755))
+	require.NoError(t, os.MkdirAll(filepath.Join(dir, "wiki"), 0o755))
+	require.NoError(t, os.Symlink(filepath.Join("..", "raw"), filepath.Join(dir, "wiki", "inwing")))
+	require.NoError(t, os.WriteFile(filepath.Join(dir, "raw", "v.md"), []byte("untouched"), 0o644))
+	require.NoError(t, os.Symlink(filepath.Join("raw", "keep"), filepath.Join(dir, "indir")))
+	require.NoError(t, os.WriteFile(filepath.Join(dir, "wiki", "exists.md"), []byte("original"), 0o644))
+	require.NoError(t, os.Symlink("nowhere.md", filepath.Join(dir, "wiki", "dangling.md")))
+	b, err := Open(dir)
+	require.NoError(t, err)
+	defer func() { _ = b.Close() }()
+
+	assert.ErrorIs(t, b.CreateRegular("wiki/inwing/facts/n.md", []byte("x")), ErrNotRegular)
+	_, err = os.Stat(filepath.Join(dir, "raw", "facts"))
+	assert.True(t, os.IsNotExist(err), "nothing created through the in-brain link")
+	assert.ErrorIs(t, b.CreateRegular("indir/../v.md", []byte("x")), ErrNotRegular)
+	got, _ := os.ReadFile(filepath.Join(dir, "raw", "v.md"))
+	assert.Equal(t, "untouched", string(got))
+
+	assert.ErrorIs(t, b.CreateRegular("wiki/exists.md", []byte("x")), fs.ErrExist)
+	got, _ = os.ReadFile(filepath.Join(dir, "wiki", "exists.md"))
+	assert.Equal(t, "original", string(got))
+	assert.ErrorIs(t, b.CreateRegular("wiki/dangling.md", []byte("x")), fs.ErrExist)
+	_, err = os.Stat(filepath.Join(dir, "wiki", "nowhere.md"))
+	assert.True(t, os.IsNotExist(err), "nothing created at the dangling link's target")
+
+	require.NoError(t, b.CreateRegular("wiki/new/facts/n.md", []byte("one")), "positive control: create with missing parents")
+	got, _ = os.ReadFile(filepath.Join(dir, "wiki", "new", "facts", "n.md"))
+	assert.Equal(t, "one", string(got))
+}