consensus(router): route inbound app-Gossip to the chain handler (α-of-K vote/cert delivery)

The α-of-K quorum vote/cert envelopes (BroadcastVote / GossipCert) ride on
p2p app-Gossip, but the handler.Op enum had no Gossip op, so the node's
inbound router dropped every GossipOp as 'unhandled message op'. The chain
handler's Gossip demux (HandleIncomingVote / HandleIncomingCert) was therefore
unreachable: no votes ingested, no cert assembled, cert-gated finality never
fired (zero Chits, frozen height) on every K>1 network.

Append a Gossip op (kept LAST so PushQuery=6 / Vote=8 / Context=10 wire bytes
are unchanged) and re-export it from networking/handler so the node can route
GossipOp -> handler.Gossip. Append-only; cert gate / VerifyWeighted untouched.
engine/chain quorum/cert/vote/finalize tests pass (CGO_ENABLED=0).

core/router/router.go

@@ -45,6 +45,11 @@ const (
 	GetContext
 	// Context is the response containing prerequisite blocks needed to verify/attach a tip
 	Context
+	// Gossip is an unsolicited app-gossip payload (e.g. the α-of-K quorum
+	// vote/cert envelope broadcast to all validators). It carries application
+	// bytes, not a consensus container, and is demuxed by the chain handler's
+	// Gossip method. MUST stay last so the preceding wire bytes are unchanged.
+	Gossip
 )
 
 // Field represents a message field

networking/handler/handler.go

@@ -46,4 +46,5 @@ const (
 	Vote                = router.Vote
 	GetContext          = router.GetContext
 	Context             = router.Context
+	Gossip              = router.Gossip
 )