Add processor count and quota APIs

[samuel-williams-shopify]

Summary

• add Process::Metrics::Processor.count as an affinity-aware integer processor count backed by Etc.nprocessors
• add Process::Metrics::Processor.quota as a floating-point capacity that accounts for cgroup v2 cpu.max limits
• inspect parent cgroups for inherited limits, cap quota at the available processor count, and fall back to the count when no finite quota is available
• document the APIs and add platform-independent and Linux-specific tests

Cgroup v1 is intentionally not supported; those systems use the safe processor-count fallback.

Testing

• bundle exec bake test
• bundle exec rubocop
• bundle exec bake decode:index:coverage lib
• forced Linux backend tests on macOS via RUBYOPT