Phase 45: pairwise ranking objective rejected - no new information, demotion artifact exposed; all axes now measured

EXPERIMENTS.md

@@ -2313,3 +2313,58 @@ a claim that nothing is left: J2W demonstrably finds 2.2 deaths per scenario,
 in well-observed data, that our ordering puts below the cut.
 
 Standing champion remains **3c7b883 at 1661.45**.
+
+## Phase 45 - the objective axis: pairwise ranking loss REJECTED (last open axis)
+
+Rationale: log-loss on a 2.3% positive rate spends its capacity on the 97%
+that obviously survive; the decision is a top-3% ordering, and the entire
+638-point gap to the leader sits at ranks 12-20. A pairwise loss (RankNet-
+style, linear scorer, within-origin death-vs-survivor difference vectors, no
+intercept) optimises exactly that boundary. Two samplers: uniform survivors,
+and hard negatives (survivors the incumbent ranks highest). Scored on recall
+at fixed N, both instruments; score drops into rank_score so p42/count/timing
+would stay byte-identical if shipped.
+
+| variant | native rec@12 | native tail 13-30 | gate rec@12 | gate tail |
+|---|---|---|---|---|
+| logbag (base) | **0.564** | 0.229 | **0.766** | 0.154 |
+| pair-uniform | 0.509 | 0.264 | 0.743 | 0.154 |
+| pair-hard | 0.518 | **0.271** | 0.746 | 0.149 |
+
+Paired at N=12: native -21 (4W/15L), gate -20 (19W/36L). Loses everywhere at
+the operating point.
+
+The apparent tail win (0.271 vs 0.229 native) motivated a head/tail hybrid:
+logbag's top-K byte-identical, pairwise re-orders only below it. Result: the
+hybrid loses at every K on both instruments (gate K12: -18 at N=16, 8W/24L;
+native K12: -12 at N=16, 4W/12L). That exposes the tail win as an artifact:
+**through rank 30 the two objectives catch the same deaths (native rec@30
+0.793 vs 0.789, gate 0.920 vs 0.894)** - pairwise does not discover new deaths
+in the tail, it demotes base's head deaths INTO the tail. When the hybrid
+keeps base's head and asks pairwise to improve the tail with its own picks,
+there is nothing there.
+
+**Conclusion: the pairwise scorer extracts no information the log-loss fit
+does not already have.** Same features, same data, same deaths found, lower
+positions. Combined with phases 41 (features), 42 (training volume), 44
+(regularisation, calibration shift), the information ceiling of this feature
+space is the champion's current ordering. Every axis is now measured:
+
+| axis | closed by |
+|---|---|
+| appetite / count | public (phase 28), break-even arithmetic (44) |
+| features | fixed-N ablation (41) |
+| training volume | public pooling (42) |
+| regularisation | C sweep, flat/negative at N=12 (44) |
+| calibration robustness | offset cannot reorder a linear model (44) |
+| objective | this phase |
+| timing / day placement | lead-time bias, coin flip (43) |
+| queue pricing | alphabetical ordering damage (43) |
+| routing-aware inclusion | phases 7/22 |
+| silent deaths | phases 6-13, 38-39 |
+
+What would beat 1661 from here is information we do not have locally: either
+a signal J2W computes that is outside this feature space entirely, or the
+forward-reading of the split file that phase 43(a) verified and declined.
+
+Standing champion: **3c7b883 at 1661.45**. Deadline 2026-08-23.