Use a monotonic clock for calibration - #301

Summary

Measure BCrypt::Engine.calibrate with Ruby's monotonic process clock instead of the adjustable wall clock.

This keeps elapsed-time measurements stable when NTP, virtualization, or an administrator moves the system clock while calibration is running, and addresses the timing fragility discussed in #235 and #248.

Reproduction

An external focused model replaces Time.now with a method that raises. The current implementation raises from calibrate; this branch returns MIN_COST for a tiny time budget because calibration no longer consults wall time.

Verification

• bundle exec rake compile spec: 39 examples, 0 failures on Ruby 4.0.6
• focused wall-clock-isolation model: pass

Compatibility

No public API, cost selection algorithm, or supported platform changes. Process.clock_gettime(Process::CLOCK_MONOTONIC) is available on the supported Ruby versions.