feat: independent fullscreen kiosk display foundation (#1)

Kiosk process owns its display lifecycle: waits for X11, keeps the
fullscreen black canvas below video, stops deterministically on signal.
Supervisor starts it before video, restarts only on process death, and
shuts the whole process group down; X11 downtime never restarts the
supervisor. Per-target opener/template abstractions for future MIPI.

internal/display/status_test.go

@@ -0,0 +1,29 @@
+package display
+
+import "testing"
+
+func TestStatusTemplateDrawsBlackCanvasWithLine(t *testing.T) {
+	canvas := &fakeCanvas{width: 800, height: 600}
+
+	NewStatusTemplate("Brak polaczenia WFB").Draw(canvas)
+
+	clears, flushes := canvas.drawCounts()
+	if clears != 1 || flushes != 1 {
+		t.Fatalf("clears=%d flushes=%d want 1 and 1", clears, flushes)
+	}
+	lines := canvas.lines()
+	if len(lines) != 1 || lines[0] != "Brak polaczenia WFB" {
+		t.Fatalf("lines=%v want [Brak polaczenia WFB]", lines)
+	}
+}
+
+func TestStatusTemplateDrawsEveryProvidedLine(t *testing.T) {
+	canvas := &fakeCanvas{width: 800, height: 600}
+
+	NewStatusTemplate("first", "second").Draw(canvas)
+
+	lines := canvas.lines()
+	if len(lines) != 2 || lines[0] != "first" || lines[1] != "second" {
+		t.Fatalf("lines=%v want [first second]", lines)
+	}
+}