feat(frost): wire interactive share-verification blame into the bundle (7.3 share-blame)

[mswilkison]

RFC-21 Phase 7.3 — interactive share-verification blame (the third fault source)

Follows PR2b-2 (the blame layer: 2a #4088 coarse f+1 evidence, 2b #4089 coordinator-equivocation proofs, 2c #4090 tests). This closes the last blame gap.

The gap

The blame layer excluded members for coarse faults and coordinator equivocation, but an interactive bad-share submitter escaped blame entirely. The engine's InteractiveAggregate surfaces candidate culprits in a typed InteractiveAggregateShareVerificationError{CandidateCulprits []uint16}, yet the runner just wrapped and returned the error, dropping them. EngineRound2ShareVerifier and Round2Collector.ClassifyCandidateCulprits were both complete but constructed/called nowhere — so a member that submitted an invalid FROST signature share was never excluded and the retry could loop.

The wiring

On a runner.Run share-verification failure, driveInteractiveRoastSigningIfEnabled (where 2b already extracts proofs):

1. errors.As the typed *InteractiveAggregateShareVerificationError (through the runner's %w wrap);
2. converts the engine's uint16 candidates → MemberIndex, dropping 0 / out-of-range so a malformed candidate can't truncate into an honest seat;
3. type-asserts the engine to Round2ShareVerifyingEngine (skip if absent — still stash 2b proofs);
4. builds an EngineRound2ShareVerifier bound to the attempt (SessionID == active.SessionID(), AttemptContextHash, TaprootMerkleRoot);
5. ClassifyCandidateCulprits re-verifies each candidate's retained share (frozen-Q1 boundary: only an ACCEPTED retained share that re-verifies INVALID is blamed; every not-the-member's-fault condition fails closed);
6. stashes the reject accusations in the same union pending-evidence entry as the 2b proofs.

BroadcastForcedSnapshot carries them; computeNextAttempt's f+1 reject gate excludes an f+1-corroborated bad-share member.

Why f+1 (not instant)

A share is self-incriminating only against the package this observer accepted — a byzantine coordinator's targeted split could otherwise make one honest observer instant-exclude an honest peer. f+1 corroboration (honest observers re-verify identical retained bytes deterministically) closes that. Only unforgeable coordinator-equivocation proofs (2b) are instant.

Safety

Best-effort + fail-safe: a non-share error, a verifier-incapable engine, malformed candidates, or an empty classification all stash nothing. EngineRound2ShareVerifier already fails closed against false blame (bound-attempt / root / submitter / package-body checks). Compile-time assertion that the cgo engine satisfies Round2ShareVerifyingEngine. Stale "evidence/proofs mutually exclusive" comments corrected (one interactive failure now carries both).

Scope / gating

Prod-dormant until the cgo interactive engine is registered (gated on the frost-secp256k1-tr audit).

Verification

• build + vet across 5 tag combos (default / frost_native / frost_roast_retry / frost_native frost_roast_retry / frost_native frost_tbtc_signer w/ CGO)
• tests: blame-fires (engine-verified-invalid share → reject through the real drive+runner+collector+verifier), skip-without-verifier, malformed-candidates-dropped; -race; gofmt clean
• design locked via Codex + Gemini consult (both PROCEED, no holes)

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