feat: Validator grid and UP/CE networking infrastructure

[yu2C]

Summary

This PR adds validator grid topology and neighbor computation aligned with JAMNP, and strengthens QUIC connection handling / stream dispatch and CE handler wire consistency. It also includes small API, naming, and index-safety cleanups so follow-up work (UP 0, automatic dialing, epoch transitions) can land cleanly.

Motivation

• JAMNP ties connectivity and UP 0 peers to grid neighbors and previous / current / next epoch validator sets; the codebase lacked a reusable grid / manager layer.
• EventBus used inconsistent keys for Subscribe vs Publish, so handlers never ran.
• ConnectionManager was not concurrency-safe (r/w on a map without a mutex).
• CE handling mixed 1-byte stream kind with an extra 4-byte length wrapper in one path, which could desynchronize reads inside handlers.
• Peer needed an accept loop, TLS-backed Ed25519 peer identity, 1-byte stream kind dispatch, and Broadcast that writes the kind byte before the payload.
• PeerSet needed multi-index lookups (key / address / validator index) and must reject malformed Ed25519 key lengths to avoid corrupt indices.
• GridMapper field names were updated to Previous / Current / Next for clearer alignment with the spec.

Scope (by file)

P0 / foundational fixes

File	Change
internal/networking/quic/event_bus.go	handlers as map[EventType][]Handler; Publish(ctx, eventType, event)
internal/networking/quic/connection.go	Mutex on ConnectionManager; Add / Remove / All

Validator grid and dial addresses

File	Change
internal/networking/validator/grid.go	GridMapper (Previous / Current / Next), ComputeWidth, same-epoch neighbors, same-index across epochs, FindIndex
internal/networking/validator/manager.go	ValidatorManager, PreferredInitiator, metadata → IPv6 + little-endian port

Peer / stream dispatch

File	Change
internal/networking/quic/peer_set.go	Multi-index map; Add returns error; Ed25519 key length must be 32 bytes when a key is present
internal/networking/quic/peer.go	Start / accept loop; validate TLS cert and extract peer key; RegisterHandler; Broadcast writes stream kind; integrates PeerSet
internal/networking/quic/handler.go	HandleStreamByKind; HandleStream keeps backward-compatible 1-byte read then dispatch

CE dispatch

File	Change
internal/networking/handler/ce/ce_handler.go	Removed incorrect outer 4-byte framing; HandleStream(blockchain, protoID, stream)

Tests

File	Change
internal/networking/quic/peer_test.go	Ed25519Key, Connect signature, broadcast wire includes kind byte
internal/networking/handler/ce/ce128_test.go, ce129_test.go	Updated Connect calls

Validation

go test ./internal/networking/quic ./internal/networking/validator ./internal/networking/handler/ce

Expected: quic and handler/ce pass; validator currently has no test files.

Out of scope (follow-ups)

• Full UP 0 block announcement state machine (handshake loop, announcement loop, duplicate UP stream resolution).
• Epoch-transition connectivity delay rules from JAMNP.
• Preferred-initiator 5s fallback and dialing all prev/current/next validators automatically.
• Restoring explicit PeerRole / builder ALPN on Connect if product wiring needs it (small follow-up PR).
• Replacing log in networking with the shared logger package for consistent levels and configuration.

[Terryhung]

Should IsNeighbor(key) consider peers whose key exists only in Previous/Next (same validator index), since JAMNP-S counts same-index-across-epochs as grid neighbours, or is Current-only lookup?

[Terryhung]

Should Broadcast follow JAMNP-S message framing (4-byte LE length after the stream kind byte)?