feat(hls): add native HLS (m3u8) download protocol (#1515)

internal/protocol/hls/doc.go

@@ -0,0 +1,24 @@
+// Package hls implements native HTTP Live Streaming downloads for m3u8
+// playlists. A task produces exactly one merged, playable output file:
+// MPEG-2 TS segments are concatenated as-is, fMP4 streams are merged as
+// init section followed by the media fragments, and AES-128 encrypted
+// segments are decrypted before they are stored.
+//
+// Supported subset:
+//   - VOD media playlists (EXT-X-ENDLIST present) and master playlists whose
+//     selected variant carries its own audio
+//   - AES-128 with the identity KEYFORMAT, explicit IV or media-sequence
+//     derived IV, including key rotation mid-playlist
+//   - EXT-X-BYTERANGE (including offset inheritance) and fMP4 init sections
+//     via a single, constant EXT-X-MAP
+//
+// Everything the merge pipeline cannot reproduce faithfully is rejected with
+// an explicit error instead of silently producing a broken output file:
+//   - live streams (no EXT-X-ENDLIST) and SAMPLE-AES / DRM encryption
+//   - EXT-X-GAP (would leave a hole) and EXT-X-DISCONTINUITY (broken timeline)
+//   - changing init sections (a repeated EXT-X-MAP is compared by all its
+//     attributes, not just the URI) and encrypted EXT-X-MAP without an
+//     explicit IV
+//   - separate audio renditions referenced by the selected variant (the
+//     merged output would have no audio) and non-identity KEYFORMATs
+package hls