test(brainfstest): swap-race asserts fail when nothing was tested

AssertNoLeakFrom and the search Query race test passed vacuously when
every symlink swap failed or every read errored. Both now count
successful symlink swaps and reads that returned the inside note and
fail if either is zero; the Query race reader no longer swallows
Query's error.

Verified by flipping: a swapper whose os.Symlink always fails turns
notes, wiki and search race tests red; Query on a missing brain dir
turns the search race test red.

Closes #177

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

ingestion/internal/brainfs/brainfstest/brainfstest.go

@@ -27,7 +27,7 @@ const outsideContent = "---\ntitle: " + Sentinel + "\naliases:\n  - " + Sentinel
 	"\n\n" + Sentinel + " [[" + Sentinel + "]]\n"
 
 // Outside writes a file outside any brain dir containing Sentinel.
-func Outside(t *testing.T) string {
+func Outside(t testing.TB) string {
 	t.Helper()
 	p := filepath.Join(t.TempDir(), "secret.md")
 	if err := os.WriteFile(p, []byte(outsideContent), 0o644); err != nil {
@@ -42,9 +42,12 @@ var swapSeq atomic.Int64
 // a symlink to outside or a FIFO into path, until the test ends. FIFOs get a
 // writer held open until the swapper exits, so a reader that skips the
 // handle check parks forever instead of reading EOF. The swapper is stopped
-// and awaited before t.TempDir cleanup.
-func StartSwap(t *testing.T, path, inside, outside string, fifo bool) {
+// and awaited before t.TempDir cleanup. It returns the number of symlinks
+// or FIFOs actually renamed into path so far, so a caller can tell a race
+// that ran from one whose every swap failed (hyperguild#177).
+func StartSwap(t testing.TB, path, inside, outside string, fifo bool) *atomic.Int64 {
 	t.Helper()
+	var swaps atomic.Int64
 	var stop atomic.Bool
 	var wg sync.WaitGroup
 	wg.Add(1)
@@ -67,22 +70,26 @@ func StartSwap(t *testing.T, path, inside, outside string, fifo bool) {
 		}()
 		for i := 0; !stop.Load(); i++ {
 			_ = os.Remove(tmp)
+			var made error
 			switch {
 			case i%2 == 0:
 				_ = os.WriteFile(tmp, []byte(inside), 0o644)
 			case fifo:
-				if err := syscall.Mkfifo(tmp, 0o644); err == nil {
+				if made = syscall.Mkfifo(tmp, 0o644); made == nil {
 					if w, err := os.OpenFile(tmp, os.O_RDWR, 0); err == nil {
 						writers = append(writers, w)
 					}
 				}
 			default:
-				_ = os.Symlink(link, tmp)
+				made = os.Symlink(link, tmp)
+			}
+			if err := os.Rename(tmp, path); err == nil && made == nil && i%2 == 1 {
+				swaps.Add(1)
 			}
-			_ = os.Rename(tmp, path)
 		}
 	}()
 	t.Cleanup(func() { stop.Store(true); wg.Wait() })
+	return &swaps
 }
 
 // WriteSentinelAt writes a file carrying Sentinel at p (creating its
@@ -107,18 +114,41 @@ func AssertNoLeak(t *testing.T, path, inside string, d time.Duration, read func(
 	AssertNoLeakFrom(t, path, inside, Outside(t), d, read)
 }
 
+// InsideMarker must appear in the inside note's content and in read's
+// output whenever read saw that note: the floor below counts such reads.
+const InsideMarker = "inside"
+
 // AssertNoLeakFrom is AssertNoLeak with the symlink pointing at target, e.g.
 // a file elsewhere in the brain written with WriteSentinelAt.
-func AssertNoLeakFrom(t *testing.T, path, inside, target string, d time.Duration, read func() string) {
+//
+// Beyond "no Sentinel in any output" it sets two floors, because without
+// them the assertion passes when nothing was tested (hyperguild#177): at
+// least one symlink must have been swapped into path, and at least one read
+// must have returned InsideMarker (a reader that always errors returns
+// nothing, and so never leaks).
+func AssertNoLeakFrom(t testing.TB, path, inside, target string, d time.Duration, read func() string) {
 	t.Helper()
-	StartSwap(t, path, inside, target, false)
-	leaks, iters := 0, 0
+	if !strings.Contains(inside, InsideMarker) {
+		t.Fatalf("inside content must contain %q", InsideMarker)
+	}
+	swaps := StartSwap(t, path, inside, target, false)
+	leaks, iters, insideReads := 0, 0, 0
 	for deadline := time.Now().Add(d); time.Now().Before(deadline); iters++ {
-		leaks += strings.Count(read(), Sentinel)
+		out := read()
+		leaks += strings.Count(out, Sentinel)
+		if strings.Contains(out, InsideMarker) {
+			insideReads++
+		}
 	}
 	if leaks > 0 {
 		t.Fatalf("outside file leaked %d times in %d iterations", leaks, iters)
 	}
+	if n := swaps.Load(); n == 0 {
+		t.Fatalf("no symlink was swapped into %s in %d iterations: the race did not run", path, iters)
+	}
+	if insideReads == 0 {
+		t.Fatalf("no read in %d iterations returned the inside note (%q): the reader read nothing", iters, InsideMarker)
+	}
 }
 
 // AssertNoHang loops read for d while a FIFO swapper (held writers) runs on