add Timer::record_scoped RAII guard

[bartoszmodelski]

Summary

• New Timer::record_scoped() method that starts the timer and returns a ScopedRecord guard
• Guard calls record_processing() automatically on drop

Usage:

fn my_function(&mut self) {
    let _guard = self.timer.record_scoped();
    // ... work ...
} // record_processing() fires here

Test plan

• cargo check -p flux-communication

🤖 Generated with Claude Code

[niki4smirn]

#[must_use]

just in case :)