refactor: extract streaming engine into internal/flinger

Move the capture/encode/send pipeline out of cmd/teleportfling into a
GUI-free Engine so the CLI and the desktop app share one implementation:
- flinger: Config/Engine/Status, audio+video+stats loops, pattern source
- config: JSON persistence at ~/.config/teleportfling/config.json
- cmd/teleportfling: thin CLI wrapper around the engine (same flags)
- golangci: extend exclusions to flinger/config

internal/flinger/flinger_test.go

@@ -0,0 +1,92 @@
+package flinger
+
+import (
+	"net"
+	"testing"
+	"time"
+)
+
+// TestEnginePatternStartStop runs the engine with the synthetic pattern source
+// and verifies it produces frames on the wire and stops cleanly.
+func TestEnginePatternStartStop(t *testing.T) {
+	cfg := DefaultConfig()
+	cfg.Source = "pattern"
+	cfg.Port = 0 // ephemeral
+
+	eng, err := New(cfg)
+	if err != nil {
+		t.Fatalf("New: %v", err)
+	}
+
+	eng.Start()
+	defer eng.Stop()
+
+	// Connect a raw receiver and read until the engine reports frames sent.
+	conn, err := net.Dial("tcp", "127.0.0.1:"+itoa(eng.sender.Port()))
+	if err != nil {
+		t.Fatalf("dial: %v", err)
+	}
+	defer func() { _ = conn.Close() }()
+
+	// The video loop sends ~30fps; wait for the counter to advance.
+	deadline := time.Now().Add(3 * time.Second)
+	_ = conn.SetReadDeadline(deadline)
+
+	// Drain whatever arrives while waiting for the frame counter to move.
+	go func() {
+		buf := make([]byte, 64*1024)
+		for {
+			if _, err := conn.Read(buf); err != nil {
+				return
+			}
+		}
+	}()
+
+	for time.Now().Before(deadline) {
+		if eng.Status().Frames > 0 {
+			break
+		}
+		time.Sleep(20 * time.Millisecond)
+	}
+
+	if eng.Status().Frames == 0 {
+		t.Error("expected frames to be counted")
+	}
+}
+
+// TestEngineConfigDefaults verifies DefaultConfig is sane.
+func TestEngineConfigDefaults(t *testing.T) {
+	c := DefaultConfig()
+	if c.Port != 9756 {
+		t.Errorf("default port = %d, want 9756", c.Port)
+	}
+	if c.Source != "screen" {
+		t.Errorf("default source = %q, want screen", c.Source)
+	}
+	if c.Quality < 1 || c.Quality > 100 {
+		t.Errorf("default quality %d out of range", c.Quality)
+	}
+}
+
+// TestNewRejectsBadSource ensures invalid sources fail fast.
+func TestNewRejectsBadSource(t *testing.T) {
+	cfg := DefaultConfig()
+	cfg.Source = "bogus"
+	if _, err := New(cfg); err == nil {
+		t.Error("expected error for unknown source")
+	}
+}
+
+func itoa(v int) string {
+	if v == 0 {
+		return "0"
+	}
+	var buf [20]byte
+	i := len(buf)
+	for v > 0 {
+		i--
+		buf[i] = byte('0' + v%10)
+		v /= 10
+	}
+	return string(buf[i:])
+}