test(frost): multi-node interactive signing e2e over the real pkg/net transport

[mswilkison]

What

The pkg/net RunnerBus adapter (broadcastChannelRunnerBus, #4080) had thorough unit coverage — seat authentication, demux, dedup, drop-on-overflow, wire round-trip — but no end-to-end test of a full interactive signing round flowing through it. This adds that.

n interactive signing runners are wired over n real pkg/net BroadcastChannel buses (one operator per seat, all on one in-memory network), each driving the deterministic fake engine, completing a whole round across the production transport adapter rather than the in-process test bus. It exercises:

• wire serialization of every RunnerMessage type (commitments, signing package, shares),
• the claimed-seat ↔ authenticated-operator-key binding,
• the per-type demux, and bounded delivery.

Two cases:

• Full-included (TestInteractiveSigningRunner_NetTransport_FullIncludedRound): group size == threshold == 2, every seat signs.
• t-of-included (TestInteractiveSigningRunner_NetTransport_ThresholdSubsetRound): group size 3, threshold 2 — the coordinator finalizes over a t-subset and the remaining committed seat observes, so the RFC-21 Phase 7.3 subset/observer flow is validated across the real transport too.

Why

This is the explicit prerequisite for retiring the coarse path ("don't delete coarse until real transport passes multi-node testing"). It's un-gated (no -tr audit dependency — fake engine, no real FROST crypto; the engine suite covers the crypto), and it raises confidence in the merged transport adapter before any production cutover.

Scope

Test-only, frost_native (pre-prod) tag; no production behavior change. The standard client-* CI does not build frost_native tags, so this is validated locally: default + frost_native build/vet, and the new tests under -race, plus the full pkg/frost/signing frost_native -race suite (no regression), gofmt clean.

🤖 Generated with Claude Code

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