test: add BFT Byzantine and storage fault fixtures

[Lchangliang]

Summary

• add an opt-in byzantine-test consensus path that emits conflicting signed proposals and typed evidence
• add disposable Docker targets for Byzantine and WAL/database corruption testing, including safe backup, restoration, and child-process supervision
• return consensus initialization errors cleanly and keep the execution-args sender alive during failed startup
• document fixture construction, authorization, operation, and isolated acceptance tests

Why

bft-jepsen needs protocol-aware and storage-aware hooks that cannot be implemented reliably with network controls alone. The hooks are compiled or packaged only into explicit test images, keeping normal and release images unchanged.

Validation

• Byzantine control/evidence unit tests: 2 passed
• conflicting-proposal construction test: 1 passed
• RUSTFLAGS='--cfg tokio_unstable' cargo check -p api --features byzantine-test
• RUSTFLAGS='--cfg tokio_unstable' cargo check -p bench
• bash docker/gravity_node/test-byzantine-fixture.sh
• bash docker/gravity_node/test-storage-fixture.sh
• targeted nightly rustfmt and git diff --check

A full macOS gravity_node check is not applicable because current main enables Tokio taskdump, which is Linux-only.