test(brainfs): pin the handle check with a FIFO held open by a writer

PR #161 review: removing ReadRegular's type check on the opened handle
stayed green, because the race test's FIFOs had no writer and read EOF
at once; with a writer holding the FIFO open, search hung after 16
iterations. The open-and-check step is split into readOpened, and
TestReadOpenedRefusesAFIFOHeldOpenByAWriter reaches it past the Lstat
with a writer open: it must return ErrNotRegular promptly (the flip is
red).

Also unit tests for ReadRegular (symlink, escape), Rel (inside incl.
..foo, escapes) and WalkRegular (regular files only), and the hard-link
residual documented where it is accepted, as in api/rawroot.go.

Refs #157

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

ingestion/internal/brainfs/brainfs.go

@@ -60,6 +60,21 @@ func (b *Brain) ReadRegular(rel string) ([]byte, error) {
 	if !fi.Mode().IsRegular() {
 		return nil, fmt.Errorf("%s: %w", rel, ErrNotRegular)
 	}
+	return b.readOpened(rel)
+}
+
+// readOpened opens rel non-blocking and reads it only if the OPENED handle is
+// a regular file. This is the check that matters under a race: an entry
+// swapped for a FIFO after the Lstat, with a writer holding it open, would
+// otherwise park io.ReadAll forever (PR #161 review: removing it hung search
+// after 16 iterations).
+//
+// Accepted residual, as in api/rawroot.go: a HARD link to an outside file is
+// a regular file and is read. Syncthing syncs contents, not links, so a peer
+// cannot create one; fs.protected_hardlinks=1 and the pod's secrets are on a
+// different filesystem from the brain volume, so a local caller cannot link
+// them in. Refusing Nlink > 1 is the next step if either changes.
+func (b *Brain) readOpened(rel string) ([]byte, error) {
 	f, err := b.root.OpenFile(rel, os.O_RDONLY|syscall.O_NONBLOCK, 0)
 	if err != nil {
 		return nil, err