perf: remodel TTFT

[Harrilee]

Remodel TTFT generation by modeling TRTLLM overlap mode (TTFT = 3x mixed step latency when bs > 4)

[Harrilee]

Preliminary Results

Commit: 46a3f23  (n=42 datapoints) [Baseline]
  TTFT APE  — mean=34.8%  median=29.7%  p90=68.8%  max=92.2%
  TPOT APE  — mean=6.6%  median=5.8%  p90=12.5%  max=17.9%

Commit: 5aefb74  (n=42 datapoints) [After ZigZag fix]
  TTFT APE  — mean=34.2%  median=29.7%  p90=58.5%  max=89.6%
  TPOT APE  — mean=6.6%  median=5.8%  p90=12.5%  max=17.9%

Commit: 923caba  (n=42 datapoints) [Current PR]
  TTFT APE  — mean=37.5%  median=35.2%  p90=61.6%  max=89.9%
  TPOT APE  — mean=6.9%  median=6.2%  p90=12.7%  max=17.9%

Will investigate on gaps on higher batch sizes