M1: X11 active-window adapter + platform fallback

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/evidence/x11_integration_test.go

@@ -0,0 +1,35 @@
+//go:build linux
+
+package evidence
+
+import (
+	"context"
+	"os"
+	"testing"
+	"time"
+)
+
+func TestX11SourceEmitsWhenDisplaySet(t *testing.T) {
+	if os.Getenv("DISPLAY") == "" {
+		t.Skip("no DISPLAY; skipping live X11 smoke test")
+	}
+	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+	defer cancel()
+
+	got := make(chan WindowSnapshot, 1)
+	go NewSource().Watch(ctx, func(s WindowSnapshot) {
+		select {
+		case got <- s:
+		default:
+		}
+	})
+
+	select {
+	case snap := <-got:
+		// Either a real window or a clean Unavailable — both are valid; we only
+		// assert the sensor produced an observation without panicking.
+		t.Logf("first snapshot: %+v", snap)
+	case <-ctx.Done():
+		t.Fatal("x11 source emitted no snapshot within timeout")
+	}
+}