feat(stream): benchmark software encode ceiling at startup

Replace the guessed transcode ceiling (CPU->1080, GPU->2160) with a measured
one. HW encoders still return 2160 instantly. A software-only host runs a
bounded encode benchmark — 3s testsrc2 through the real libx264 superfast
settings at 1080/720/480, top-down — and reports the rung it sustains at
>=1.5x realtime (margin for real decode + busier content).

Fixes risk 2: a weak NAS/old CPU that is ffmpeg-capable but can't keep up
with a 1080p software encode no longer advertises a 1080 ceiling, so
decideStreamPlan routes oversized sources to an external player instead of a
stuttering transcode. Floors at 480; each probe is timeout-bounded so a
wedged ffmpeg can't stall daemon startup.

internal/engine/encode_benchmark_test.go

@@ -0,0 +1,52 @@
+package engine
+
+import (
+	"context"
+	"os/exec"
+	"testing"
+)
+
+func TestBenchmarkMaxTranscodeHeight_HardwareSkipsProbe(t *testing.T) {
+	// Hardware encoders return 2160 without touching ffmpeg — pass a bogus path
+	// to prove no subprocess runs.
+	for _, hw := range []HWAccel{HWAccelNVENC, HWAccelQSV, HWAccelVAAPI, HWAccelVideoToolbox} {
+		got := BenchmarkMaxTranscodeHeight(context.Background(), "/nonexistent/ffmpeg", hw)
+		if got != 2160 {
+			t.Errorf("hw=%s: got %d, want 2160", hw, got)
+		}
+	}
+}
+
+func TestBenchmarkMaxTranscodeHeight_NoFFmpegKeepsDefault(t *testing.T) {
+	if got := BenchmarkMaxTranscodeHeight(context.Background(), "", HWAccelNone); got != 1080 {
+		t.Errorf("empty ffmpeg path: got %d, want 1080 (historical default)", got)
+	}
+}
+
+func TestBenchmarkMaxTranscodeHeight_SoftwareReturnsValidRung(t *testing.T) {
+	ffmpeg, err := exec.LookPath("ffmpeg")
+	if err != nil {
+		t.Skip("ffmpeg not on PATH — software benchmark needs a real encoder")
+	}
+	got := BenchmarkMaxTranscodeHeight(context.Background(), ffmpeg, HWAccelNone)
+	switch got {
+	case 1080, 720, 480:
+		// any rung is valid; the exact one depends on the host's CPU.
+	default:
+		t.Errorf("software ceiling = %d, want one of {1080,720,480}", got)
+	}
+}
+
+func TestMeasureEncodeRealtimeFactor_RealEncoder(t *testing.T) {
+	ffmpeg, err := exec.LookPath("ffmpeg")
+	if err != nil {
+		t.Skip("ffmpeg not on PATH")
+	}
+	factor, ok := measureEncodeRealtimeFactor(context.Background(), ffmpeg, benchmarkRung{height: 480, width: 854})
+	if !ok {
+		t.Fatal("480p probe failed to run on a host with ffmpeg")
+	}
+	if factor <= 0 {
+		t.Errorf("realtime factor = %.2f, want > 0", factor)
+	}
+}