Publish partial messages

[lucassaldanha]

Implements the wire-level partial-message send/receive flow (Steps 3 + 4 of #435).

Scope

Inbound dispatch (Step 3)

Replace the stub at GossipRouter.kt:476 (processPartialMessageExtension) with the full flow:

1. Validate that local + peer both advertised ControlExtensions.partialMessages.
2. Get-or-create GroupState(topic, groupId), mark as peer-initiated.
3. Enforce DoS caps (tracked in Refine DoS protection and other security/performance concerns #449).
4. Call handler.onIncomingRpc(from, peerStates, rpc).

Outbound publish (Step 4)

Add Gossip.publishPartial(topic, groupId, actions) entry point:

1. Hop to pubsub event thread.
2. Get-or-create GroupState, reset TTL.
3. Populate peerStates with zero-values for mesh peers (fallback to fanout when mesh is empty).
4. Invoke PublishActionsFn, iterate yielded actions.
5. Per action: build Rpc.PartialMessagesExtension{topicID, groupID, partialMessage?, partsMetadata?}, enqueue through GossipRpcPartsQueue (do NOT bypass — PR Add partial message support for gossipsub #433 got this wrong).
6. Apply nextPeerState atomically.
7. Enforce spec MUST: omit partialMessage when peer supports sending partial but did not request it on this topic.

Not in scope here

Routing-layer changes (suppress full-msg, suppress IDONTWANT, replace IHAVE) are tracked separately — see the routing-integration sub-issue of #435.

Reference

Design: docs/partial-messages.md §4.3, §5, §6.2 (lands with the first PR on #435).
go-libp2p: extensions.go:221, partialmsgs.go:216, 303.