feat: log structured reconnect diagnostics for kiosk display layers (#2)

cmd/groundstation-display/main_test.go

@@ -1,7 +1,9 @@
 package main
 
 import (
+	"errors"
 	"testing"
+	"time"
 
 	"git.piksar.eu/piotr/groundstation/internal/display"
 )
@@ -83,3 +85,41 @@ func TestBuildTargetRejectsUnknownTarget(t *testing.T) {
 		t.Fatal("unknown target must fail")
 	}
 }
+
+func TestFormatDiagnosticRendersStructuredFields(t *testing.T) {
+	got := formatDiagnostic(display.Diagnostic{
+		Target:     "x11",
+		Layer:      "fallback-canvas",
+		Event:      display.EventOpenFailed,
+		Err:        errors.New("connect X11: refused"),
+		RetryDelay: 500 * time.Millisecond,
+	})
+	want := `target=x11 layer=fallback-canvas event=open_failed error="connect X11: refused" retry_delay=500ms`
+	if got != want {
+		t.Fatalf("formatDiagnostic=%q want %q", got, want)
+	}
+}
+
+func TestWireDiagnosticsTagsEveryLayerWithItsName(t *testing.T) {
+	layers, err := buildTarget("x11")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	wireDiagnostics("x11", layers)
+
+	for _, layer := range layers {
+		if layer.Options.Target != "x11" {
+			t.Fatalf("layer %q target=%q want x11", layer.Name, layer.Options.Target)
+		}
+		if layer.Options.Layer != layer.Name {
+			t.Fatalf("layer %q diagnostic layer=%q", layer.Name, layer.Options.Layer)
+		}
+		if layer.Options.Diagnostic == nil {
+			t.Fatalf("layer %q must carry the reconnect diagnostic hook", layer.Name)
+		}
+	}
+
+	// The shared hook must be safe to call from any layer goroutine.
+	layers[0].Options.Diagnostic(display.Diagnostic{Target: "x11", Layer: layers[0].Name})
+}