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/pipeline/root_test.go

@@ -213,5 +213,5 @@ func TestBuildAndWriteNeverWritesThroughAPageSwappedForASymlink(t *testing.T) {
 	}
 	got, _ := os.ReadFile(target)
 	assert.Equal(t, "untouched", string(got), "buildAndWrite wrote through a swapped-in symlink")
-	assert.Positive(t, ok, "positive control: some runs wrote the page")
+	assert.Positive(t, ok, "positive control: some runs returned without error")
 }