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/vectorstore/sync_root_test.go

@@ -42,6 +42,11 @@ func TestSyncRefusesSymlinkedNotesAndSurvivesSwapRaces(t *testing.T) {
 	brain := t.TempDir()
 	writeNote(t, brain, "wiki/w/facts/ok.md", "inside note")
 	require.NoError(t, os.Symlink(brainfstest.Outside(t), filepath.Join(brain, "wiki", "w", "facts", "leak.md")))
+	// A link to a file elsewhere in the brain: os.Root allows it, only the
+	// regular-file checks refuse it (hyperguild#171). raw/ is not a scan root,
+	// so its sentinel can only arrive through the link.
+	inBrain := brainfstest.WriteSentinelAt(t, filepath.Join(brain, "raw", "s.md"))
+	require.NoError(t, os.Symlink(filepath.Join("..", "..", "..", "raw", "s.md"), filepath.Join(brain, "wiki", "w", "facts", "inlink.md")))
 
 	emb := &recordingEmbedder{}
 	store := &stubStore{}
@@ -51,6 +56,7 @@ func TestSyncRefusesSymlinkedNotesAndSurvivesSwapRaces(t *testing.T) {
 	assert.NotContains(t, emb.all(), brainfstest.Sentinel)
 	for p := range store.upserts {
 		assert.NotContains(t, p, "leak.md")
+		assert.NotContains(t, p, "inlink.md")
 	}
 
 	p := filepath.Join(brain, "knowledge", "r.md")
@@ -61,6 +67,7 @@ func TestSyncRefusesSymlinkedNotesAndSurvivesSwapRaces(t *testing.T) {
 		return e.all()
 	}
 	brainfstest.AssertNoLeak(t, p, "inside", 1500*time.Millisecond, sync)
+	brainfstest.AssertNoLeakFrom(t, p, "inside", inBrain, 1500*time.Millisecond, sync)
 	brainfstest.AssertNoHang(t, p, "inside", 1500*time.Millisecond, 6*time.Second, func() { _ = sync() })
 }