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

internal/protocol/hls/testdata/media/README.md

@@ -0,0 +1,10 @@
+Media fixtures generated locally with FFmpeg 7.0.2 (testsrc + sine sources,
+no third-party content):
+
+- sample-00.ts / sample-01.ts: one continuous MPEG-2 TS encode split at a
+  packet boundary, so continuity counters and PCR are consistent across the
+  segment join exactly like segments cut from a single real-time stream.
+- fmp4-init.mp4: ftyp+moov init section of a fragmented MP4.
+- fmp4-seg1.m4s .. fmp4-seg3.m4s: moof+mdat media fragments (styp omitted by
+  the muxer, which is legal) split at moof boundaries from one fragmented MP4
+  (-movflags +frag_keyframe+empty_moov+default_base_moof -frag_duration 500000).