bench: retune spin A/B sweep for saturation (4/6 cores, 16 loadgen)

The old 1/2/4/8 sweep with an 8-core loadgen plateaued ~102k rps at 4 and 8
server cores -- loadgen-bound, ~85% server CPU, so the 4-core (gate-active)
point was never actually saturated. Retune:

- CORES 1 2 4 8 -> 1 2 4 6. The gate is active at 2-4, inert at 1 and >=5, so
  6 is now the saturating inert control and 4-vs-6 brackets the gate boundary
  under real load (8 couldn't be saturated).
- LOADGEN_CPUS 12-19 (8c) -> 8-23 (16c); WRK_THREADS 8 -> 16; WRK_CONNS
  256 -> 512. Gives >=2x loadgen-cores-per-server-core at every sweep point
  (16:1/8:1/4:1/2.7:1) and leaves cores 6-7 for the OS.
- analyzer: gate label now follows the real 2..=4 rule (6 reads inert), and
  the reading guide notes to confirm the 6-core control is CPU- not
  loadgen-bound before trusting it.

Note: at 22/24 logical CPUs in use, loadgen will share SMT siblings with the
server; sibling-disjoint isn't achievable at 16 loadgen cores. Override
LOADGEN_CPUS after checking lscpu -e if that matters.

analyze_spin_ab_urus.py

@@ -18,7 +18,10 @@ import os
 import re
 import sys
 
-GATED = {2, 4}  # core counts where the RFC 004 spin policy is active
+def gate_active(cores):
+    """RFC 004 spinner cap is non-zero only at 2..=4 schedulers (1 and >=5
+    allow 0 spinners), so the policy is active exactly there."""
+    return 2 <= cores <= 4
 
 
 def parse_latency_ms(s):
@@ -88,7 +91,7 @@ def main():
         b = fnum(before.get(k, {}).get("sustained_rps"))
         a = fnum(after.get(k, {}).get("sustained_rps"))
         spd = f"{a / b:.2f}x" if (a and b) else "—"
-        gate = "ACTIVE" if cores in GATED else "inert"
+        gate = "ACTIVE" if gate_active(cores) else "inert"
         bs = f"{b:,.0f}" if b is not None else "—"
         as_ = f"{a:,.0f}" if a is not None else "—"
         print(f"{srv:<10} {scn:<4} {cores:>5} {bs:>12} {as_:>12} {spd:>8}  {gate}")
@@ -107,7 +110,7 @@ def main():
                 delta = f"{(a - b):+.2f}"
             else:
                 delta = "—"
-            gate = "ACTIVE" if cores in GATED else "inert"
+            gate = "ACTIVE" if gate_active(cores) else "inert"
             bs = f"{b:.2f}" if b is not None else "—"
             as_ = f"{a:.2f}" if a is not None else "—"
             print(f"{srv:<10} {scn:<4} {cores:>5} {bs:>10} {as_:>10} {delta:>9}  {gate}")
@@ -126,8 +129,10 @@ def main():
 
     print(
         "\nReading it: spin is 'worth it' if s3 (and/or s2) RPS rises and/or "
-        "tail latency drops at cores 2/4 (gate ACTIVE), with cores 1/8 (inert) "
-        "staying flat. Flat-or-worse at 2/4 = not worth it on real traffic."
+        "tail latency drops at the gate-ACTIVE cores (2-4), with the inert "
+        "controls (1 and >=5, e.g. 6) staying flat. Flat-or-worse at 2/4 = "
+        "not worth it on real traffic. Cross-check that the saturating control "
+        "(6 cores) is actually CPU-bound, not loadgen-bound, before trusting it."
     )
     if "axum-mem" in {k[0] for k in keys}:
         print(