Add processor_count and processor_quota APIs

[samuel-williams-shopify]

This adds two processor-capacity APIs without changing the behavior of Etc.nprocessors:

• Etc.processor_count is an alias for Etc.nprocessors and returns an Integer.
• Etc.processor_quota returns the usable processor capacity as a Float. On Linux it accounts for cgroup v1/v2 CPU bandwidth quotas, capped by the affinity-aware processor count; otherwise it falls back to Etc.processor_count.to_f.

The cgroup lookup resolves the process membership through /proc/self/cgroup and /proc/self/mountinfo, considers limits inherited from ancestor cgroups, and falls back safely when cgroup information is unavailable or malformed.

This implements the separate fractional API discussed in Feature #21797, while preserving Etc.nprocessors compatibility, and is an alternative API shape to #75.

Tests cover the aliases and return types, cgroup v1 and v2 fractional quotas, inherited limits, unlimited/malformed values, escaped mount paths, and Ractor use.

Tested with:

rake test
# 24 tests, 4637 assertions, 0 failures, 0 errors

rake build
# etc 1.4.6 built to pkg/etc-1.4.6.gem

[samuel-williams-shopify]

After experimenting with this code, I wonder if we shouldn't bother supporting cgroups v1. It's more complexity and every system where it's relevant is using cgroups v2 now.