moq-lite: make compression group-scoped (one DEFLATE stream per group)

Rework payload compression so a whole group is compressed as a single
DEFLATE stream rather than each frame independently. Within a group the
frames already share one ordered, reliable QUIC stream, so the group —
not the frame — is the random-access unit; compressing across it gains
cross-frame redundancy and compresses the framing too, while the first
frame of a group stays independently decodable and groups remain
mutually independent.

- Publisher Compression in TRACK_INFO stays a boolean end-to-end hint.
- SETUP Compression parameter is now a presence flag (capability), not
  an algorithm list.
- A per-group Compression field (0 verbatim, 1 DEFLATE) is carried in
  GROUP, in datagram bodies, and in a reintroduced minimal FETCH_OK
  (present only when the hint is set). The per-frame Compression field
  is gone.
- DEFLATE is the only scheme; others are left to future extensions, one
  in effect per hop.
- Security note updated: the CRIME/BREACH window is now per group, not
  per frame.

This supersedes the earlier per-frame, enum-based compression on this
branch.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W8bLV6vHzucLNvDhPk3bMP

Author: claude