Fix block buffer readiness and execution epoch handling

[Lchangliang]

Summary

• add a ready wait helper for BlockBufferManager that uses Notify::enable to avoid lost wakeups
• route all readiness wait sites through the helper
• use the epoch carried by reth execution results when setting compute results
• add focused BlockBufferManager readiness tests

Dependency

This PR depends on gravity-reth #360 because gravity_node now reads ExecutionResult::epoch from greth. I validated this branch with a local patch to /Users/lightman/repos/grevm-reth; after #360 lands, this PR should bump the greth rev in bin/gravity_node/Cargo.toml / Cargo.lock and can be marked ready.

Tests

• cargo +nightly fmt --all
• RUSTFLAGS='--cfg tokio_unstable' cargo test -p block-buffer-manager
• LC_ALL=en_US.UTF-8 LANG=en_US.UTF-8 RUSTFLAGS='--cfg tokio_unstable' cargo check -p gravity_node --config 'patch."https://github.com/Galxe/gravity-reth".greth.path="/Users/lightman/repos/grevm-reth"'

Audit coverage

• fix(consensus): prevent 'Parent block not found' panic on sync + rebuild #681
• chore: gitignore Claude Code harness local state #698