Layer 1 complete: E3-E6 + E2 analysis add-ons

Finishes the Layer 1 analytical core. All six experiments run with honest,
publication-quality figures; 71 tests green.

- E3 region-matched grounding: `grounding.exercised` knob + per-region tail
  survival. Matched holds the exercised region's tail (0.49) where uniform
  spreads thin and lets it collapse (0.07).
- E4 multi-teacher recombination: `run_coverage` runner. Union coverage matches
  U(K_T,rho,q) exactly. Finding: mean-mixture distillation shows NO surviving
  benefit (a conservation law — 1/K_T dilution cancels the union gain); a
  union-preserving max-merge (M2N2-style) does. E4 reports both operators.
- E5 QD vs greedy: greedy drives fixation (H~0.01); QD holds H at 0.48-0.88,
  rising with the novelty exponent.
- E6 re-mint gate: `arm` multi-override sweep. Re-minting a collapsed lineage
  locks in divergence of KL-to-original; gating on diversity prevents it.
- E2 analysis add-ons (from the companion work order, numbers verified): new
  analysis.py (reduce_to_stationary, critical_grounding with bootstrap CI ->
  g*=0.048, 95% CI [0.047,0.050]); tail_band_metrics + per-band logging; the
  E2 figure rebuilt as a 2x2 (defined g*+CI, g=0 flagged as a finite-time
  artifact, tail item-vs-mass, per-rarity-band panel). Uses truth-mass-weighted
  tail coverage rather than the raw (martingale) tail_mass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

configs/layer1/E4.yaml

@@ -0,0 +1,34 @@
+experiment: E4_multiteacher_decorrelation
+kind: coverage
+seed: 20260704
+n_replicates: 200
+
+# Multi-teacher recombination (blueprint 2.5-E4 / 2.7.1). Build K_T teachers with exact
+# marginal retention q and pairwise retention-correlation rho, form the pupil from their
+# mixture (n draws total = matched budget), and report TWO coverages:
+#   union_coverage    -> construction-level U(K_T,rho,q) (must match the closed form)
+#   surviving_coverage-> tail items that survive the pupil's size-n resampling (+ grounding)
+# Expect: both rise with K_T and (1-rho); at rho=1 many teachers give no benefit over one;
+# the union-surviving gap shrinks as grounding g rises.
+truth:
+  K: 500
+  R: 1
+  tail: zipf
+  zipf_s: 1.1
+  tail_frac: 0.5
+  tail_threshold: 2.0e-3
+
+coverage:
+  n: 300          # pupil sample size (matched budget across teachers)
+  q: 0.5          # per-teacher marginal tail retention
+
+sweep:
+  - param: K_T
+    values: [1, 2, 3, 5]
+  - param: rho
+    values: [0.0, 0.25, 0.5, 0.75, 1.0]
+  - param: g
+    values: [0.0, 0.02, 0.05]
+
+output:
+  dir: results/E4