compression: clarify the SETUP list is decode capability only

The Compression parameter/option lists algorithms an endpoint can
decompress; it does not advertise what it can produce. When sending,
an endpoint compresses with an algorithm the receiver advertised.
Resolves recurring ambiguity now that the publisher names the algorithm
in the track property (so the receiver reads it rather than computing a
mutual selection).

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

Author: claude

draft-lcurley-moq-compression.md

@@ -65,8 +65,8 @@ COMPRESSION Setup Option {
 ~~~
 
 **Algorithm**:
-One or more Algorithm identifiers (see [Compression Algorithms](#compression-algorithms)) that the sender can decompress, each a varint, filling the Option Value.
-The identifier `none` (0) MUST NOT be listed (it requires no negotiation); an endpoint lists whichever other algorithms it supports.
+One or more Algorithm identifiers (see [Compression Algorithms](#compression-algorithms)) that the sender can decompress, each a varint, filling the Option Value — its decode capability. An endpoint does not advertise which algorithms it can *produce*; when sending, it compresses with one the receiver advertised.
+The identifier `none` (0) MUST NOT be listed (it requires no negotiation).
 An endpoint that does not support the extension omits the option.
 
 A sender MUST NOT compress with an algorithm the receiver did not advertise, and MUST NOT compress before it has received the receiver's COMPRESSION option.

draft-lcurley-moq-lite.md

@@ -631,7 +631,7 @@ The remaining bindings convey the path in their own handshake.
 A relay MUST NOT forward the Path Parameter; like other per-hop setup metadata it applies only to this hop (see [Session](#session)).
 
 ### Compression Parameter {#compression-parameter}
-The Compression Parameter advertises the payload compression [algorithms](#compression) the sender can *decompress* on this hop.
+The Compression Parameter advertises the payload compression [algorithms](#compression) the sender can *decompress* on this hop — its decode capability. An endpoint does not advertise which algorithms it can *produce*; when sending, it compresses with one the receiver advertised.
 The Parameter Value is a sequence of algorithm identifiers, each a variable-length integer, packed back-to-back to fill the Parameter Length.
 The identifier `none` (0) MUST NOT be listed. An endpoint that does not support compression omits the parameter.