test(brainfs): per-caller in-brain symlink targets for slices 1-3 (#171)

os.Root refuses links that leave the brain on its own, so the slice 1-3
callers' outside-target tests stayed green when a caller read without
brainfs's Lstat/SameFile checks (PR #168 round-2 review). Adds a static
in-brain link (wiki/w/facts/inlink.md -> ../../../raw/s.md) and an
AssertNoLeakFrom swap race with an in-brain target for search.Query,
hydrate, vectorstore.Sync, readExcerpt and api.ReadNote.

Flip-verified: replacing each caller's bfs.ReadRegular with an
os.OpenRoot(brainDir).ReadFile turns every one red on the new assertions
(Query, hydrate, Sync, readExcerpt, ReadNote).

Also from that review: rename TestReadRegularNever... to
...DoesNotReturn... (rule 9: a sampled race), and AssertNoLeakFrom's
failure message names the symlink target instead of 'outside file'.

Closes #171

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

ingestion/internal/brainfs/brainfs_test.go

@@ -218,7 +218,7 @@ func TestReadDirRegularListsOnlyRegularFilesAndRefusesSymlinkedComponents(t *tes
 // A final component swapped for a symlink to another regular file in the
 // brain between the Lstat and the open: os.Root follows in-root links, so
 // only comparing the two (os.SameFile) refuses it.
-func TestReadRegularNeverReturnsAnInBrainFileSwappedInBehindASymlink(t *testing.T) {
+func TestReadRegularDoesNotReturnAnInBrainFileSwappedInBehindASymlink(t *testing.T) {
 	dir := t.TempDir()
 	require.NoError(t, os.WriteFile(filepath.Join(dir, "s.md"), []byte("zqsamefilesentinel"), 0o644))
 	p := filepath.Join(dir, "r.md")