Add clock uncertainty/jitter margin to timing checks

[robtaylor]

Summary

Add a configurable clock uncertainty margin to timing checks, matching commercial STA set_clock_uncertainty behavior.

Current State

GEM timing checks use exact clock period with no pessimism margin. Real designs have PLL jitter, duty cycle distortion, and other clock uncertainty that must be accounted for.

Proposed Approach

• Add --clock-uncertainty <ps> CLI flag (default 0)
• Setup check becomes: slack = clock_period - uncertainty - data_arrival - setup_time
• Hold check becomes: slack = data_arrival - hold_time - uncertainty
• Support separate setup/hold uncertainty values

Impact

Medium — adds realism to timing checks without major architectural changes.

Effort

Low — single parameter addition to existing check logic.