Why: Q3 WP5 requires a same-batch dual-run harness (direct vs squad
compiled graph) that records differences without demanding byte-identical
output, feeding DecideSwitch evidence for the runtime.entry default-switch
decision.
Evidence: eval/ab_test.go mechanics (theme-based scoring, status/theme/
latency diff notes, P50, DecideSwitch wiring, RunFunc error tolerance,
empty batch); wfcompile TestABDemoBriefDualRun runs the demo.brief scenario
on both entries (direct = scripted single-agent answer; squad = real
RunCompiled with stub backend and judge) and logs the report: both sides
100/100, summary-form difference recorded, decision '切换条件已满足…合入前
需要产品确认'. Full backend suite green (SELF); CI authoritative (GATE).
Impact: new eval/ab.go harness (pure mechanics, no runtime dependency);
no product default is flipped - runtime.entry stays direct until product
confirmation per the DecideSwitch report semantics (evidence recorded in
the task card and PR).
Refs: Q3 plan WP5 (2026-10-07-agent-stack-q2q3-plans.md); task card
.task/feature-agent-stack-q3.md
