test: add runtime contract regression checks

[mrcrgl]

Summary

• add a regression test that enforces the documented ProcessManager::add() pre-start panic contract
• assert panic text to keep the contract explicit and stable
• keep issue #28 runtime contract coverage current in changelog

Validation

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

Closes #28