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/api/readnote_root_test.go

@@ -23,6 +23,13 @@ func TestReadNoteRefusesSymlinksAndSurvivesSwapRaces(t *testing.T) {
 	_, body, _, err := api.ReadNote(brain, "wiki/w/facts/leak.md")
 	assert.Error(t, err)
 	assert.NotContains(t, body, brainfstest.Sentinel)
+	// A link to a file elsewhere in the brain: os.Root allows it, only the
+	// regular-file check refuses it (hyperguild#171).
+	brainfstest.WriteSentinelAt(t, filepath.Join(brain, "raw", "s.md"))
+	require.NoError(t, os.Symlink(filepath.Join("..", "..", "..", "raw", "s.md"), filepath.Join(facts, "inlink.md")))
+	_, body, _, err = api.ReadNote(brain, "wiki/w/facts/inlink.md")
+	assert.Error(t, err)
+	assert.NotContains(t, body, brainfstest.Sentinel)
 
 	p := filepath.Join(facts, "r.md")
 	require.NoError(t, os.WriteFile(p, []byte("---\nx: y\n---\ninside"), 0o644))
@@ -39,5 +46,7 @@ func TestReadNoteRefusesSymlinksAndSurvivesSwapRaces(t *testing.T) {
 		return s
 	}
 	brainfstest.AssertNoLeak(t, p, "---\nx: y\n---\ninside", 1500*time.Millisecond, read)
+	inBrain := brainfstest.WriteSentinelAt(t, filepath.Join(brain, "raw", "s.md"))
+	brainfstest.AssertNoLeakFrom(t, p, "---\nx: y\n---\ninside", inBrain, 1500*time.Millisecond, read)
 	brainfstest.AssertNoHang(t, p, "---\nx: y\n---\ninside", 1500*time.Millisecond, 6*time.Second, func() { _ = read() })
 }