Profile fixed PD workload pressure against trace anchor

runs/frontier-fidelity-envelope-v1/fixed-pd-pressure-matching-card.md

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+# EXP-SIMFID-Q30-FIXED-PD-PRESSURE: calibrate a nontrivial fixed-shape workload
+
+> Status: planned. This is a real-only workload calibration; its measurements
+> are not part of a Frontier-versus-vLLM winner comparison.
+
+## Question
+
+The former Fixed-PD workload (`2048 -> 128`, 0.215 request/s/GPU) was nearly
+single-request at TP4: its real mean E2E was 604 ms at a TP4 cluster rate of
+0.86 request/s, or about 0.52 in-flight requests by Little's law. It therefore
+does not test the decode batching regime of Trace-PD.
+
+Can a no-prefix, uniform `4096 -> 256` workload be assigned a fixed offered
+rate that reaches the healthy Trace-PD operating point without using a
+simulator result for calibration?
+
+## Controlled calibration
+
+The reference is the real Trace-PD `TP4/MNS64` cell, chosen before this probe
+because it is the real TTFT/E2E winner on that surface:
+
+| Reference metric | Target |
+|---|---:|
+| mean TTFT | 245.95 ms |
+| mean TPOT | 13.18 ms |
+| mean E2E | 44.99 s |
+| offered rate | 0.215 request/s/GPU; 0.86 request/s at TP4 |
+| in-flight proxy | 38.69 requests |
+
+The probe fixes Qwen3-30B-A3B BF16, community vLLM 0.20.0, H20, TP4, MNS64,
+MBT=8192, chunked prefill, and prefix caching off. It compares per-GPU rates
+`{1, 2, 3, 4}` (global TP4 rates `{4, 8, 12, 16}` request/s). Every rate has
+257 exact `4096 -> 256` requests in each of three fresh-server trials; rate
+orders are rotated across trials.
+
+## Decision rule
+
+Pool the three trials for each rate. Among rates with every request completed,
+choose the rate minimizing the Euclidean distance of the two *relative* errors
+for mean TTFT and mean TPOT from the above targets. Report p90 TTFT/TPOT/E2E,
+trial variation, and `global_rate * mean_E2E` as an in-flight proxy, but do not
+turn any of them into an SLO. The selected rate becomes a frozen workload
+contract. The subsequent 12-cell real/simulator surface must be fresh and is
+not allowed to reuse this calibration data.
+
+## Interpretation boundary
+
+This selects a workload regime, not a simulator parameter and not a serving
+capacity point. It may make Fixed-PD more comparable to Trace-PD in batching
+pressure, but it deliberately continues to exclude trace-shaped arrivals and
+prefix reuse; those are separately evaluated workload dimensions.