Timing summary report: WNS/TNS/WHS/THS metrics

[robtaylor]

Summary

Add standard timing signoff metrics to simulation output: Worst Negative Slack (WNS), Total Negative Slack (TNS), Worst Hold Slack (WHS), Total Hold Slack (THS).

Current State

GEM reports individual setup/hold violations via event buffer but doesn't aggregate them into the standard metrics that timing engineers look at first.

Proposed Approach

• Accumulate per-DFF worst slack across all simulation cycles
• At end of simulation, report:
  • WNS: Worst (most negative) setup slack across all DFFs
  • TNS: Sum of all negative setup slacks
  • WHS: Worst hold slack
  • THS: Sum of all negative hold slacks
  • Number of violating endpoints (setup and hold separately)
• Support JSON output format for CI integration
• Optional: histogram of slack distribution

Impact

Medium — these are the first numbers any timing signoff engineer checks. Essential for usability.

Effort

Low — aggregate existing violation data that's already collected.