feat(stream): burn bitmap (PGS/DVB) subtitles into the video via overlay

Bitmap subs can't be served as WebVTT, so the user picks one and the daemon
re-encodes with it overlaid. HLSSessionConfig.BurnSubtitleIndex (*int, nil=no
burn) flows into the cache key + a -filter_complex graph:
  [0✌️0]<vchain>[base];[0:s:N][base]scale2ref[sub][base2];[base2][sub]overlay[vout]
Overlay after the tonemap (SDR subs keep brightness); scale2ref fits the PGS
canvas to the output. Invalid/text/out-of-range index -> clean-encode fallback.
IsTextSubtitle now includes "text" (parity with the web classifier).

internal/engine/hls_cache_smoke_test.go

@@ -98,7 +98,7 @@ func TestHLSCacheSmoke(t *testing.T) {
 	encodeDur := time.Since(t0)
 	t.Logf("session 1: MISS completed in %s", encodeDur.Round(time.Millisecond))
 
-	key := cache.KeyFor(source, "720p", 0)
+	key := cache.KeyFor(source, "720p", 0, -1)
 	if !cache.HasComplete(key) {
 		t.Fatalf("cache.HasComplete(%s) is false after successful encode", key)
 	}