1788: add multistream policies

[liam]

What Changed

• Added AssociationID field to MsgVersion for multistream peer identification
• Added MsgCreateStream (createstrm) and MsgStreamAck (streamack) message types for establishing additional TCP streams within an association
• Added StreamType enum (General, Data1-Data4) and MaxAssociationIDLen constant
• Added createstrm/streamack to makeEmptyMessage() command dispatch

Why It Was Necessary

• Bitcoin SV nodes support the https://github.com/bitcoin-sv-specs/protocol/blob/master/p2p/multistreams.md, which allows a single logical peer connection to use multiple
  TCP sockets for different traffic types (e.g., separating block data from transaction traffic via the BlockPriority stream policy). go-wire had no support for the
  wire-level messages needed to negotiate and establish these additional streams.

Testing Performed

• 14 new unit tests across 3 test files: msg_create_stream_test.go (6 tests), msg_stream_ack_test.go (5 tests), msg_version_test.go (3 new tests)
• Tests cover: encode/decode roundtrips, empty/oversized association IDs, wire errors, makeEmptyMessage dispatch, and backward compatibility (decoding old-format version
  messages without AssociationID)
• All existing tests continue to pass
• End-to-end verified against a Bitcoin SV node with -multistreams=1 -multistreampolicies=BlockPriority,Default, confirming successful association negotiation, DATA1
  stream creation via createstrm/streamack, and block sync over the secondary stream

Impact / Risk

• Backward compatible: AssociationID in MsgVersion is only written when non-empty, so encoded messages are identical to the previous format for peers that don't use
  multistreams. Decoding tolerates the field being absent.
• No changes to existing message types or their encoding beyond the optional trailing field in MsgVersion
• New message types are additive only

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[mrz1836]

LGTM