Calibration demo

The vllm-vcr inspect subcommands include a calibration harness for quick local checks. The synthetic demo trace is deliberately heavy-tailed, so it shows the difference between the trace-fitted model and simple mean/std-dev knobs.

The harness checks two properties:

1. TraceLatency replay reproduces source-trace quantiles within tolerance.
2. KnobLatency cannot reproduce heavy tails: its [0.3*mean, 1.7*mean] clamp caps p99/p50 at roughly 1.7x for any knob settings.

This model-level check applies to ITL and to TTFT on unloaded traces. On loaded captures, the TTFT marginal comes from queueing and chunk interference rather than a sampled distribution, so this check can fail by design. Loaded TTFT is checked by the arrival-replay scenarios below.

# 1. Generate a synthetic heavy-tailed trace (lognormal TTFT/ITL).
cargo run --bin vllm-vcr -- inspect gen-demo -o /tmp/demo.jsonl

# 2. Model-level calibration (no transport).
cargo run --bin vllm-vcr -- inspect calibrate /tmp/demo.jsonl

# 3. Wire-level: start the simulator and measure client-side.
cargo run --bin vllm-vcr -- inspect calibrate-e2e /tmp/demo.jsonl --requests 60

--fast on gen-demo produces a small-magnitude trace for quick e2e testing (TTFT ~15-40ms, ITL ~3-10ms). All subcommands accept --json for machine-readable output and --seed for determinism.