fix(playback): probe real VideoToolbox modes, fix chapterthumbs extraction

Address adversarial review: the VideoToolbox probe now smoke-encodes
both encoders in the exact constant-quality (-q:v) mode the uncapped
transcode path emits, so Intel Macs without qscale support resolve
auto to software instead of approving unrunnable commands; and
chapter-thumbnail extraction gains a videotoolbox case (hardware
decode, software filters) instead of misclassifying CPU args as a
hardware attempt — HDR thumbnails on macOS now tone-map via the shared
CPU chain.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/playback/gpudetect.go

@@ -202,9 +202,12 @@ func ffmpegSupportsVideoToolbox(ffmpegPath string) (bool, string) {
 }
 
 // probeFFmpegVideoToolbox verifies the configured FFmpeg exposes VideoToolbox
-// decode plus both encoders, then smoke-encodes one frame. No filter probes:
-// the videotoolbox pipeline keeps decoded frames in system memory, so the
-// regular software filter graph applies (see appendHWAccelArgs).
+// decode plus both encoders, then smoke-encodes one frame per encoder in the
+// same constant-quality mode the uncapped transcode path emits (-q:v needs an
+// Apple Silicon compression session; Intel Macs must fail here so auto
+// resolves to software instead of approving commands that cannot run). No
+// filter probes: the videotoolbox pipeline keeps decoded frames in system
+// memory, so the regular software filter graph applies (see appendHWAccelArgs).
 func probeFFmpegVideoToolbox(ffmpegPath string) nvencProbeResult {
 	if output, err := runFFmpegProbe(ffmpegPath, "-hide_banner", "-hwaccels"); err != nil {
 		return nvencProbeResult{reason: "hwaccels probe failed: " + probeFailure(err, output)}
@@ -220,18 +223,27 @@ func probeFFmpegVideoToolbox(ffmpegPath string) nvencProbeResult {
 		return nvencProbeResult{reason: "hevc_videotoolbox encoder unavailable"}
 	}
 
-	if output, err := runFFmpegProbe(ffmpegPath,
-		"-hide_banner",
-		"-loglevel", "error",
-		"-f", "lavfi",
-		"-i", "testsrc2=size=640x360:rate=1",
-		"-frames:v", "1",
-		"-an",
-		"-c:v", "h264_videotoolbox",
-		"-f", "null",
-		"-",
-	); err != nil {
-		return nvencProbeResult{reason: "h264_videotoolbox smoke encode failed: " + probeFailure(err, output)}
+	for _, smoke := range []struct {
+		encoder string
+		quality string
+	}{
+		{"h264_videotoolbox", "65"},
+		{"hevc_videotoolbox", "60"},
+	} {
+		if output, err := runFFmpegProbe(ffmpegPath,
+			"-hide_banner",
+			"-loglevel", "error",
+			"-f", "lavfi",
+			"-i", "testsrc2=size=640x360:rate=1",
+			"-frames:v", "1",
+			"-an",
+			"-c:v", smoke.encoder,
+			"-q:v", smoke.quality,
+			"-f", "null",
+			"-",
+		); err != nil {
+			return nvencProbeResult{reason: smoke.encoder + " smoke encode failed: " + probeFailure(err, output)}
+		}
 	}
 
 	return nvencProbeResult{available: true}

internal/playback/gpudetect_test.go

@@ -376,6 +376,38 @@ func TestResolveHWAccelWithFFmpegDarwinRequiresBothVTEncoders(t *testing.T) {
 	}
 }
 
+func TestResolveHWAccelWithFFmpegDarwinFallsBackToNoneWhenSmokeEncodeFails(t *testing.T) {
+	setupHWAccelTest(t)
+	currentGOOS = "darwin"
+	ffmpeg := writeFakeFFmpeg(t, fakeFFmpegProbe{videotoolbox: true, h264VT: true, hevcVT: true})
+
+	if got := ResolveHWAccelWithFFmpeg("auto", ffmpeg.path); got != "none" {
+		t.Fatalf("ResolveHWAccelWithFFmpeg() = %q, want none when smoke encode fails", got)
+	}
+}
+
+func TestVideoToolboxProbeSmokesBothEncodersInConstantQualityMode(t *testing.T) {
+	setupHWAccelTest(t)
+	currentGOOS = "darwin"
+	ffmpeg := writeFakeFFmpeg(t, successfulVideoToolboxProbe())
+
+	if got := ResolveHWAccelWithFFmpeg("auto", ffmpeg.path); got != "videotoolbox" {
+		t.Fatalf("ResolveHWAccelWithFFmpeg() = %q, want videotoolbox", got)
+	}
+	log, err := os.ReadFile(ffmpeg.logPath)
+	if err != nil {
+		t.Fatalf("read probe log: %v", err)
+	}
+	// The smoke encodes must exercise the exact uncapped (-q:v) mode the
+	// transcode arg builder emits — Intel Macs fail that mode at session
+	// creation, so probing without it would approve unrunnable commands.
+	for _, want := range []string{"-c:v h264_videotoolbox -q:v 65", "-c:v hevc_videotoolbox -q:v 60"} {
+		if !strings.Contains(string(log), want) {
+			t.Fatalf("probe log missing %q:\n%s", want, log)
+		}
+	}
+}
+
 func TestExplicitVideoToolboxBypassesFFmpegProbe(t *testing.T) {
 	setupHWAccelTest(t)