refactor: rename inspect to replay across the codebase

Renames inspect-ui/ to replay-ui/, internal/inspect/ to internal/replay/,
the CLI subcommand from `sanderling inspect` to `sanderling replay`, and
updates all references in docs, Makefile, README, and Go comments.

internal/replay/watcher_test.go

@@ -0,0 +1,54 @@
+package replay
+
+import (
+	"testing"
+)
+
+func TestWatcher_UnsubscribeRemovesChannel(t *testing.T) {
+	w := NewWatcher(t.TempDir())
+	first := w.Subscribe()
+	second := w.Subscribe()
+	third := w.Subscribe()
+
+	if count := len(w.subscribers); count != 3 {
+		t.Fatalf("expected 3 subscribers, got %d", count)
+	}
+
+	w.Unsubscribe(second)
+
+	if count := len(w.subscribers); count != 2 {
+		t.Fatalf("expected 2 subscribers after Unsubscribe, got %d", count)
+	}
+
+	// broadcast should still notify remaining subscribers
+	w.broadcast()
+	select {
+	case <-first:
+	default:
+		t.Error("first subscriber did not receive broadcast")
+	}
+	select {
+	case <-third:
+	default:
+		t.Error("third subscriber did not receive broadcast")
+	}
+}
+
+func TestWatcher_UnsubscribeUnknownChannelIsNoop(t *testing.T) {
+	w := NewWatcher(t.TempDir())
+	existing := w.Subscribe()
+
+	stranger := make(chan struct{})
+	w.Unsubscribe(stranger)
+
+	if count := len(w.subscribers); count != 1 {
+		t.Fatalf("expected 1 subscriber after no-op Unsubscribe, got %d", count)
+	}
+
+	w.broadcast()
+	select {
+	case <-existing:
+	default:
+		t.Error("existing subscriber did not receive broadcast")
+	}
+}