feat(plugins): lifecycle event bus — observability, error recording, recovery

[MerhiOPS]

Summary

• add a lifecycle bus with subscribe/unsubscribe support, structured lifecycle events, production observability snapshots, capped error history, retry, and auto-disable recovery
• route all plugin lifecycle mutations through manager-owned transition/failure paths and forward lifecycle events into the backend runtime via volt:plugins
• add lifecycle bus, recovery, snapshot-count, and backend runtime tests covering ordering, filtering, error retention, and recovery flows

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace
• pnpm typecheck
• pnpm test