fix: support custom runtime control message injection

[mrcrgl]

Summary

• add RuntimeHandle::control(RuntimeControlMessage) for arbitrary control-message forwarding
• add RuntimeHandle::custom(T) convenience helper for custom payload injection
• make RuntimeGuard::handle() return Arc<RuntimeHandle> (concrete handle) while preserving existing trait-object usability via coercion
• add RuntimeGuard::control(...) and RuntimeGuard::custom(...) helpers
• add integration test proving custom payloads are delivered through runtime control path
• update changelog under Unreleased -> Fixed

Why

Issue #20 reports that RuntimeControlMessage::Custom cannot be injected because runtime handles only expose shutdown/reload pathways.

Validation

• cargo fmt --all -- --check
• cargo clippy --all-features -- -D warnings
• cargo test --all
• cargo test --no-default-features

Closes #20