docs: partial-messages extension design

[lucassaldanha]

Summary

Adds docs/partial-messages.md — the design document for the forthcoming gossipsub partial-messages extension implementation tracked under #435.

Landing the doc on develop ahead of any implementation so that:

• Sub-issues of Add support for partial messages extension #435 can link to a stable anchor.
• Reviewers of later implementation PRs have a shared reference for scope, API shape, and deferred items.
• The design can be amended via PRs to this file (with a decision-log entry) as the spec evolves — the spec is lifecycle 1A and expected to change.

Contents

Nine sections:

1. Scope and non-goals (MVP bounds, deferred items).
2. Reference pins (libp2p/specs commit 6b6203ee, Prysm prysm/partial-cells-current @ e8480a86, plus related in-flight spec PRs to watch).
3. jvm-libp2p ↔ client responsibility boundary table.
4. Public API — PartialMessagesHandler<PeerState>, PublishAction (with nextPeerState), PublishActionsFn, PartialMessagesPeerFeedback.
5. Routing rules — three integration points inside GossipRouter.
6. State and lifecycle — GroupState, TTL in heartbeats, DoS caps matching go-libp2p defaults.
7. Known gaps vs. full spec (incl. interop-test-client future checklist).
8. Implementation plan — 10-step checklist mirrored into Add support for partial messages extension #435.
9. Decision log — append-only.
10. References (links)

Non-goals for this PR

• No code changes. This is a design anchor only.
• Not pinned to a specific commit of go-libp2p-pubsub yet — that will be added to the decision log when the first implementation PR lands.

Test plan

• n/a — docs-only.