test: Go simnet harness for charon/pluto e2e duty tests

[varex83agent]

What

Adds test-infra/harness/, a Go test harness that runs simnet-style end-to-end attester duty tests over distributed-validator clusters mixing charon nodes (in-process, reusing charon's own app.Run) and pluto nodes (subprocesses of the pluto binary), plus a CI workflow.

Charon v1.7.1 (our parity reference) is consumed as a regular Go module dependency — no fork. The harness reuses its cluster.NewForT deterministic fixtures, testutil/beaconmock, testutil/validatormock and testutil/relay directly.

Design

                relay (in-process libp2p)
                beaconmock (shared chain state, deterministic duties)
                    ▲ Go              ▲ HTTP (per-node gateway + submission capture)
             charon app.Run      pluto run / charon run (subprocess)
             (in-process)             ▲ validator API
                                 validatormock (harness-driven over HTTP)

• Per-node HTTP beacon gateways over one shared beaconmock. Charon's beaconmock implements dynamic behaviour (duties, validators, attestation data) only on its Go client interface; its HTTP server serves static stubs. The gateway fronts the shared mock over real HTTP so external processes get a complete beacon API, and captures each node's submissions for assertions.
• Real p2p partial-signature exchange. Upstream simnet uses an in-memory ParSigEx transport; the harness omits it so out-of-process nodes participate in QBFT consensus and threshold signing.
• Assertions at the beacon boundary: for some slot, every node must submit an attestation, and all payloads must be identical after JSON normalization (group-signed threshold aggregates). The in-process baseline additionally uses upstream-style BroadcastCallback assertions.

Scenarios

Test	Cluster	Status
TestSimnetAttesterCharonInProcess	3× charon, in-process bmock	✅ passes (~5s)
TestSimnetAttesterCharonViaGateway	3× charon via HTTP gateway	✅ passes (~4s)
TestSimnetAttesterPluto	3× pluto	⏭ skips until pluto run lands
TestSimnetAttesterMixed	2× charon + 2× pluto, threshold 3	⏭ skips until pluto run lands

The pluto scenarios probe $PLUTO_BIN run --help and activate automatically once the run command exists. The subprocess path is already validated end-to-end today: pointing PLUTO_BIN at a charon binary (which accepts the same flags) passes the mixed scenario in ~5s, covering the on-disk fixture layout, run flags, readiness polling, HTTP-driven validator mock and capture assertions.

CI

.github/workflows/go-harness.yml:

• charon-simnet: charon-only scenarios, no Rust build — fast signal.
• pluto-simnet: builds pluto-cli (same steps as dkg-runner) and runs the full suite with PLUTO_BIN set.

Notes

• The two replace directives in go.mod are copied from charon's own go.mod (Go does not propagate a dependency's replaces); they must move in lockstep when bumping the pinned charon version.
• pluto run requirements for the scenarios to activate are documented in the README (charon-equivalent flags + validator API on the configured address).
• Follow-ups: proposer/sync-committee scenarios (direct translation of upstream simnet's beaconmock options) and below-threshold fault injection.

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