Read Linux RSS directly from procfs

[OskarEichler]

Summary

OS.rss_bytes currently starts ps for every POSIX call even though Linux exposes the current process resident set in /proc/self/status.

This parses VmRSS (reported in kB) directly on Linux and preserves the existing ps path when procfs is unreadable or lacks the field.

Verification

• added a focused procfs regression
• Ruby 4.0.6: 37 examples, 0 failures
• combined candidates: 42 examples, 0 failures on Ruby 4.0.6 and 3.2.11
• syntax and diff checks pass

macOS/BSD and Windows/JRuby behavior is unchanged.

[OskarEichler]

A second pass found a small TOCTOU edge case between File.readable? and File.foreach: if procfs disappears or becomes unreadable in between, RSS collection would raise instead of preserving the existing ps fallback. I added a narrow SystemCallError fallback and regression coverage. The focused specs pass (23 examples).