fix: verify gossiped tx proposer signature before mempool entry

[0xMayoor]

HandleGossipNonce (server.go:560) does RecoverAddress + slot-owner match before storing. the sibling HandleGossipTxs (server.go:584) doesn't — txs land via OnTxsReceived → mempool.AddTx with only a sender-membership check.

a byzantine group member POSTs a forged MsgValidationVote{InferenceId: target, VoterSlot: <victim's own slot>, ProposerSig: garbage}. it passes membership auth, enters the victim's mempool, and the victim's collectValidationJobs (host.go:775) calls hasMempoolValidationOrVote which trips purely on slot match — no proposer-sig check. the host skips its honest validation for that inference. same shape for MsgRevealSeed via maybeRevealSeed's early-return at host.go:675-677.

never evicted: bad-sig tx never applies (applyValidationVote machine.go:862 rejects via verifyProposerSig), so mempool.RemoveIncluded never sees it; persists for the whole session.

restricting gossip to group members doesn't close this — the route is already members-only. a byzantine member passes auth and still serves a forged inner tx. orthogonal to content verification.

Fix: add StateMachine.VerifyGossipedTx mirroring the existing per-tx verifyProposerSig calls in applyValidation, applyValidationVote, applyRevealSeed. expose a Host.VerifyGossipedTx wrapper. filter at ingestion in HandleGossipTxs (server.go:584) — drop bad-sig txs before passing to OnTxsReceived, same shape as HandleGossipNonce. regression in gossip_tx_verify_test.go with reject-forged + accept-valid pairs for each of the three affected tx kinds, plus unknown-slot and pass-through-other-kinds guards.

related: #1311 (same class on the GET /sessions/:id/signatures path), both introduced by #1284.