Add processor metrics

[samuel-williams-shopify]

Summary

• add Process::Metrics::Processor for per-process interval CPU measurements
• report fractional utilization, where one occupied core is 1.0 and multi-core usage can exceed 1.0
• expose stable process start times from the Linux /proc and portable ps backends so PID reuse establishes a new baseline
• document the interval semantics and add deterministic coverage for dynamic process sets and invalid deltas

The existing stateless General#processor_utilization percentage remains unchanged. Processor::Sample#utilization is an interval ratio suitable for direct use as ORCA cpu_utilization.

Testing

• bundle exec sus
• bundle exec rubocop
• bundle exec bake decode:index:coverage lib
• Linux Ruby 3.3 container smoke test against /proc