add q08 risk-limit calibration evidence (A/B results)

queue/designs/q08_risk_limit_ab.md

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+# QUEUE-08 — Risk-limit A/B re-validation: $5M liquidity floor on the exp-26 reference
+
+**Status:** QUEUED · **Priority:** P1 · **Effort:** tool-only (no new code)
+
+## Hypothesis (prove)
+The $5M liquidity floor improves net IR and cuts drawdown on the **post-reset**
+reference signal (pre-reset exp 18, EVIDENCE#008: net IR 0.81→0.98, cumDD
+7.93%→5.44%), while size/concentration caps hurt by cutting deployed capital.
+Needs re-validation on the exp-26 lineage because exp 18 is pre-clean-lake and
+not comparable (EVIDENCE#009/010). Source: `book/CLAIMS.md` open question +
+`book/README.md` `TODO(evidence-needed: reconciliation of exp 18 risk-limit spec
+on the post-reset reference signal)`.
+
+## Change vs exp-26 reference (ONE variable)
+- Reference: the saved exp-26 prediction (run `21afc6af…`, mlflow exp 25).
+- A/B via `rd_risk_calibrate` (runs limit-vs-no-limit A/B + sensitivity grid
+  over size_cap_pct, concentration_cap_pct, liquidity_floor_adv) and/or
+  `rd_backtest` with `risk_limits` on the SAME saved `pred.pkl`:
+  - baseline: no limits (this must reproduce the exp-26 net +2.13% / IR 0.21);
+  - candidate: `{"liquidity_floor_adv": 5000000, "size_cap_pct": 0.12,
+    "concentration_cap_pct": 0.95, "drawdown_pause_pct": 0.10}` (round-3 spec).
+- Pick the spec (B2 calibration) that keeps live ≈ backtest.
+
+## Acceptance
+- Candidate spec: `net_IR > 0.21` AND `net_max_drawdown < 7.69%` vs no-limit on
+  the same pred. Size/concentration caps expected to REDUCE deployed capital
+  (record the direction as confirmation of exp 18).
+- If the floor is a no-op (gates don't bind at this signal) → report that gates
+  are no-ops when the signal is the bottleneck (exp 20 pattern) as a PROVEN
+  clean-lake result.
+
+## Execution prerequisites
+- None (uses saved pred + `rd_risk_calibrate`/`rd_backtest`). Trace the A/B as
+  an experiment; record the spec chosen for the next live round.