feat: add restart supervisor with backoff

[mrcrgl]

Summary

• add a built-in RestartSupervisor runnable that supervises a child runnable and restarts it on failure
• add configurable exponential backoff settings via RestartBackoff
• keep supervisor responsive to shutdown/reload while child runs and while waiting in backoff
• add examples/restart_supervisor.rs and document it in README/example docs
• include restart_supervisor in CI example runs
• update integration tests and changelog entries for the new naming
• remove the unreleased RestartWrapper alias

Validation

• cargo fmt --all -- --check
• cargo clippy --all-features -- -D warnings
• cargo test --all
• cargo test --no-default-features
• timeout 30s cargo run --example simple
• timeout 45s cargo run --example dynamic_add
• timeout 30s cargo run --example restart_supervisor

Closes #19