fix: make custom runtime control messages clone-safe

[mrcrgl]

Summary

• change RuntimeControlMessage::Custom payload from Box<dyn Any + Send + Sync> to Arc<dyn Any + Send + Sync>
• derive Clone for RuntimeControlMessage and remove panic-based manual clone implementation
• update runtime custom-message helpers to construct Arc payloads
• add regression test ensuring cloning custom control messages is safe
• update changelog under Unreleased -> Fixed

Why

Issue #27 reports that cloning RuntimeControlMessage::Custom panics. This breaks expected clone semantics and can cause runtime surprises in generic code paths.

Validation

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

Closes #27