feat(app): implement simnet integration

[iamquang95]

Wires in-process simnet support into pluto run (--simnet-beacon-mock /
--simnet-validator-mock), so a full all-pluto cluster runs the complete duty
pipeline — schedule → QBFT consensus → threshold sign → broadcast — against mock
beacon/validator clients, with no real beacon node or VC. Mirrors Charon's
app.Run simnet wiring; this is what the compose smoke harness
(TestSmoke/all_pluto) drives.

Changes

• Start the ported BeaconMock + validatormock inside run: point both beacon clients at the in-process mock, load share keystores for the validator mock, and drive it from the scheduler via a new abstract SlotTickFn seam (keeps wire.rs decoupled from the mock type).
• --p2p-relays="" now means "no relays" (Charon parity) — enables isolated/offline runs.
• Beacon-mock endpoint coverage for the whole flow: POST validators, block produce/publish, sync-committee duties/contribution, and attestation / sync-committee pool submits; the validator mock now sends wire-format SingleAttestation and dials loopback (not the 0.0.0.0 bind address).
• Log-level parity with Charon so the smoke alert gate can pass: quiet simnet-only noise (libp2p relay churn, force_direct, weak-KDF keystores, expired-duty QBFT drops).

Testing

• cargo +nightly fmt --all --check and clippy --workspace --all-targets --all-features -D warnings clean; workspace tests green.
• New in-repo tests: single-node boot, validator resolution, produce-block, attestation-shape, and the --simnet-* bridge.
• End-to-end duty flow (4 nodes, threshold 3) exercised via the compose smoke harness.