viser v0.10.0

HDR10 static-metadata preservation: PQ/HLG encodes now carry mastering-display
colour volume (SMPTE ST 2086) and content light level (MaxCLL/MaxFALL) through
to the output on both x265 and SVT-AV1, not just the transfer/primaries tags.

Added

• viser-ffmpeg::hdr — Hdr10Metadata, MasteringDisplay, and
  probe_hdr10_metadata(). Reads the first video frame's side_data_list via
  ffprobe and normalises chromaticity/luminance into the integer units x265's
  master-display / max-cll parameters consume.
• SourceFormat.hdr10 and SourceFormat::enrich_hdr10() — attaches
  probed HDR10 static metadata to HDR sources (no-op for SDR). Wired into the
  per-title, per-segment, and delivery pipelines.
• x265 HDR10 signalling — encode_color_args() emits master-display=… and
  max-cll=… (MasteringDisplay::to_x265_string) for HDR sources carrying
  static metadata.
• SVT-AV1 HDR10 signalling — emits mastering-display=… / content-light=…
  via -svtav1-params, using SVT-AV1's real-valued chromaticity/luminance
  grammar (MasteringDisplay::to_svtav1_string) rather than x265's scaled
  integers. Repeated -svtav1-params (rate-control + HDR) are coalesced into a
  single flag so neither set is dropped.
• FATE tests — HDR10 metadata extraction and encode round-trip on both x265
  and SVT-AV1 (fate_probe_extracts_hdr10_metadata,
  fate_encode_preserves_hdr10_metadata,
  fate_svtav1_encode_preserves_hdr10_metadata).