fix(hls): enforce supported HLS subset with explicit errors

The merge pipeline concatenates stored bytes, so features it cannot
reproduce faithfully are now detected and rejected with a clear error
instead of silently succeeding with a broken output:

- EXT-X-GAP (a hole in the merged file) and EXT-X-DISCONTINUITY
  (a broken timeline)
- separate audio renditions referenced by the selected variant: the
  master EXT-X-MEDIA groups are parsed and an AUDIO group with its own
  URI fails the resolve; unreferenced groups are ignored
- changing init sections: a repeated EXT-X-MAP is compared by all of
  its attributes (URI and BYTERANGE), not just the URI
- encrypted EXT-X-MAP without an explicit IV: the spec does not define
  a sequence-derived IV for init sections
- non-identity AES-128 KEYFORMAT values

The supported matrix is documented in the package doc.

Add regression tests for every rejection plus real media validation:
ffmpeg-generated TS segments and fMP4 init/fragments are committed as
fixtures, downloaded through the fetcher (including an AES-128 key
rotation variant), and the merged outputs are validated structurally -
TS on the 188 byte grid with continuity counter rules, PAT/PMT and PES
presence, fMP4 by walking the ISO-BMFF box hierarchy.

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).