Gossip routing integration for partial messages

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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 #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](../blob/develop/docs/partial-messages.md) (lands with the first PR on #435).

Read docs/partial-messages.md §5 and trace the three routing paths in GossipRouter: broadcastInbound/broadcastOutbound, IDONTWANT emission, and gossipsub heartbeat lazy-push. Compare the referenced go-libp2p sections and inspect GossipRpcPartsQueue, partialMessagesEnabled(), and onEmitGossip. Done means all three specified routing changes honor peer handshake and partial-message state without changing behavior for other peers.

Written by the indexing model from the issue text.