Review peer scoring impact of partial messages

Nobody has claimed this yet.

## Scope split

**MVP (in scope here):**
- Wire the `PartialMessagesPeerFeedback.reportFeedback(topic, peer, kind)` side-channel into the existing gossip scoring infrastructure.
- `INVALID` feedback hooks into `notifyRouterMisbehavior` (the same path used by the existing v1.3 Control-Extensions handshake penalty).
- `USEFUL` / `IGNORED` feedback: decide whether these should affect score, or remain informational for now. Lean informational for MVP unless evidence suggests otherwise.

**Deferred (post-MVP, pending spec guidance):**
- Defining partial-message-specific scoring rules (spec is currently silent; go-libp2p also does not score partial-message traffic).
- Penalties for protocol-level misbehaviour specific to partial messages (e.g. inconsistent metadata, metadata regression, excessive metadata-only spam).

Revisit once the libp2p spec adds scoring guidance or we see concrete interop issues.

## Reference
Design: [`docs/partial-messages.md` §4.4, §7.2](../blob/develop/docs/partial-messages.md) (lands with the first PR on #435).
go-libp2p: no partial-specific scoring in `score.go` at the reference pin.

Start with docs/partial-messages.md §4.4 and §7.2, then trace PartialMessagesPeerFeedback.reportFeedback(topic, peer, kind) into the existing gossip scoring infrastructure and notifyRouterMisbehavior. Confirm how the v1.3 Control-Extensions handshake penalty is wired. Done means INVALID feedback uses that path, while USEFUL and IGNORED remain informational unless the evidence supports scoring them.

Written by the indexing model from the issue text.