Add support for partial messages extension

@lucassaldanha is already working on this.

Since Feb 11, 2026.

## Summary

Add support for the [gossipsub partial-messages extension](https://github.com/libp2p/specs/blob/master/pubsub/gossipsub/partial-messages.md) (spec lifecycle 1A, merged via [libp2p/specs#685](https://github.com/libp2p/specs/pull/685)).

Target consumer: Teku (PeerDAS / cell-level dissemination).

## Design

Full design document lives in-repo at [`docs/partial-messages.md`](../blob/develop/docs/partial-messages.md) (lands with the first PR on this issue). Highlights:

- jvm-libp2p is a transport + per-peer bookkeeper for opaque partial-message RPCs; the client (Teku) owns encoding of `partsMetadata`, generation of `groupID`, merging, validation, reassembly, and completion detection. `partsMetadata` and `partialMessage` are fully opaque to the library.
- Client-facing API: `PartialMessagesHandler` (generic per-peer state), `publishPartial(topic, groupId, actions)`, `PublishAction(partialMessage?, partsMetadata?, nextPeerState?)`, plus a `PartialMessagesPeerFeedback` side-channel for peer scoring.
- Routing integration: suppress full-message send to partial-requesting peers, suppress IDONTWANT to peers we request partial from, replace IHAVE with `onEmitGossip` for partial-capable peers during heartbeat.
- Per-group lifecycle with TTL (in heartbeats) + DoS caps on peer-initiated groups (defaults: 255 per topic, 8 per (topic, peer)), matching go-libp2p.

## Scope

**In scope (MVP):**
- Full wire protocol: `SubOpts.requestsPartial`/`supportsSendingPartial`, `RPC.partial`, both metadata-only and payload-only variants in both directions.
- Client plug-in API (handler, publish, peer feedback).
- Routing rule integration (3 points in `GossipRouter`).
- Group lifecycle + DoS caps.

**Deferred (documented in §7 of the design doc):**
- `interop-test-client` partial-messages support.
- Partial-specific peer-scoring rules beyond the existing handshake penalties.
- Topic-level "partial-only" mode (spec explicitly defers).
- Reassembling a full `Message` back into the normal gossip flow.

## Implementation checklist

Each step is tagged with the sub-issue that tracks it. Tests are folded into each sub-issue's done-criteria; the final mixed-peer simulator scenario is tracked as a done-criterion on #456.

- [ ] 1. Per-topic `SubOpts` flag plumbing (outbound announce + inbound parse + coercion + ignore-on-unsubscribe) — #444
- [ ] 2. `PartialMessagesHandler`, `PublishAction`, `PublishActionsFn`, `GroupState` container — #446
- [ ] 3. Inbound `RPC.partial` dispatch (replace `GossipRouter.kt:476` stub with full flow) — #445
- [ ] 4. Outbound `publishPartial(...)` through `GossipRpcPartsQueue` (not bypassing) — #445
- [ ] 5. End-to-end integration test with a trivial bitmap-based handler — done-criterion on #445
- [ ] 6. Routing: full-message suppression for partial-requesting peers — #456
- [ ] 7. Routing: IDONTWANT suppression — #456
- [ ] 8. Heartbeat tick + TTL GC + DoS caps + cleanup hooks (peer disconnect, topic unsubscribe) — #449
- [ ] 9. Routing: IHAVE replacement with `onEmitGossip` — #456
- [ ] 10. Simulator scenario + mixed-peer interop test (partial + non-partial nodes on same topic) — done-criterion on #456

### Cross-cutting sub-issues

- #447 — `PartialMessagesPeerFeedback` wiring (in scope, Step 2) + partial-specific scoring rules (deferred).
- #448 — "Eager push" pattern (primarily a Teku-side concern; API must support it — verification within Step 2 / #446).

## References

- Spec: [libp2p/specs `pubsub/gossipsub/partial-messages.md`](https://github.com/libp2p/specs/blob/master/pubsub/gossipsub/partial-messages.md) (merge commit `6b6203ee`).
- Reference impl: [libp2p/go-libp2p-pubsub](https://github.com/libp2p/go-libp2p-pubsub) (`extensions.go`, `partialmessages/partialmsgs.go`, `gossipsub.go`, `pubsub.go`).
- Client reference: [OffchainLabs/prysm `prysm/partial-cells-current`](https://github.com/OffchainLabs/prysm/tree/prysm/partial-cells-current).
- Superseded earlier attempt: #433 (closed).