test(watcher): a marker link planted during the run is refused and not retried (#158)

Review round 2 on PR #195: the marker writes had no test, so reverting
them to by-path writes, or the refusal to log-only, stayed green. The
link is planted from inside the fake model call, between the walk's
Lstat and the marker write, for both markers and both an outside and
an in-brain relative target; red on all four flips. The marker-entry
skip is now checked for .failed too.

Also pins cleanRel's acceptance of a backslash as a filename byte in
brainfs itself, and stops the buildAndWrite control claiming writes it
does not count.

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

ingestion/internal/brainfs/brainfs_test.go

@@ -529,3 +529,20 @@ func TestCreateRegularCreatesOnlyNewFilesThroughRealDirectories(t *testing.T) {
 	got, _ = os.ReadFile(filepath.Join(dir, "wiki", "new", "facts", "n.md"))
 	assert.Equal(t, "one", string(got))
 }
+
+// On Linux a backslash is an ordinary filename byte, not a separator, so a
+// name containing one stays a single in-root component (hyperguild#158: the
+// watcher retried such a note forever while cleanRel refused it).
+func TestReadAndPutRegularTreatABackslashAsAFilenameByte(t *testing.T) {
+	dir := t.TempDir()
+	b, err := Open(dir)
+	require.NoError(t, err)
+	defer func() { _ = b.Close() }()
+	require.NoError(t, b.PutRegular(`a\..\b.md`, []byte("x")))
+	got, err := os.ReadFile(filepath.Join(dir, `a\..\b.md`))
+	require.NoError(t, err, "the name was not kept as one component")
+	assert.Equal(t, "x", string(got))
+	got, err = b.ReadRegular(`a\..\b.md`)
+	require.NoError(t, err)
+	assert.Equal(t, "x", string(got))
+}