Gossip routing integration for partial messages

[lucassaldanha]

Implements the three spec-mandated routing changes inside GossipRouter (Steps 6, 7, 9 of #435).

All three are gated on partialMessagesEnabled() and on the per-peer handshake state established in #444.

Scope

1. Full-message suppression (Step 6)

When broadcasting a Message for topic T to peer P:

• If peerSupportsPartialMessages(P) AND peerRequestsPartial(P, T) → do not send the full message to P. The client pushes parts via publishPartial(...).
• Filter applied in broadcastInbound / broadcastOutbound, before messages are queued into GossipRpcPartsQueue.
• Spec MUST: if peer supports sending partial but did NOT request, we still send the full message. Omitting partialMessage when sending a partial RPC to such a peer is handled in Publish partial messages #445.

2. IDONTWANT suppression (Step 7)

When emitting IDONTWANT for a message on topic T:

• If we iRequestPartial(T) AND peerSupportsSendingPartial(P, T) → skip IDONTWANT to P.
• go-libp2p: gossipsub.go:892-904.

3. IHAVE replacement with onEmitGossip (Step 9)

During gossipsub heartbeat lazy-push:

• Partition IHAVE targets into fullPeers and partialPeers = { p | iSupportSendingPartial(T) ∧ peerRequestsPartial(p, T) }.
• Do not enqueue IHAVE for partialPeers.
• For each locally-initiated group under T, call handler.onEmitGossip(T, groupId, partialPeers, peerStatesForGroup) once per group.
• go-libp2p: gossipsub.go:2018-2074.

Reference

Design: docs/partial-messages.md §5 (lands with the first PR on #435).