moq-lite: drop the per-group compression toggle (flagless)

With the shared per-group DEFLATE stream and the RFC 7692 trailing-bytes
trim, per-frame overhead is ~1 byte, so a per-group on/off toggle isn't
worth a wire field. Compression is now decided purely by the end-to-end
Publisher Compression signal plus per-hop negotiation.

- Publisher Compression stays an end-to-end signal ("good candidate for
  compression") that fans out unchanged, so a hop that doesn't compress
  still forwards it for a downstream hop to act on.
- Remove the per-group Compression field from GROUP and datagram bodies,
  and remove FETCH_OK (it only carried that field). A receiver infers
  compression from the signal plus its own SETUP advertisement.
- Spell out the RFC 7692 trim (strip the redundant 00 00 FF FF, since we
  frame the slices ourselves) and that the decoder keeps one context per
  group.

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

Author: claude