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>

figures/plot_E6.py

@@ -0,0 +1,76 @@
+"""E6 figure: the re-minting gate and irreversibility.
+
+Re-minting freezes the current distribution as the new grounding reference and discards
+the original truth. Re-minting a collapsed lineage locks in the collapse: KL to the
+original truth diverges, because the lost original tails can no longer be grounded.
+Gating re-mint on diversity refuses to re-mint while collapsed and keeps KL bounded;
+re-minting a healthy lineage is harmless. Usage:  python figures/plot_E6.py [results/E6]
+"""
+
+from __future__ import annotations
+
+import sys
+from pathlib import Path
+
+import matplotlib.pyplot as plt
+import numpy as np
+
+sys.path.insert(0, str(Path(__file__).parent))
+from _figlib import load_bundle, savefig  # noqa: E402
+
+STYLE = {
+    "healthy_remint":     ("#2ca02c", "re-mint while healthy (H high)"),
+    "collapsed_remint":   ("#d62728", "re-mint while collapsed (ungated)"),
+    "collapsed_gated":    ("#1f77b4", "collapsed + diversity gate"),
+    "collapsed_noremint": ("#7f7f7f", "collapsed, no re-mint (baseline)"),
+}
+
+
+def main(results_dir: str = "results/E6") -> None:
+    df, cfg = load_bundle(results_dir)
+    period = cfg["dynamics"]["remint"]["period"]
+    G = cfg["generations"]
+    remint_gens = list(range(period, G + 1, period))
+
+    fig, axes = plt.subplots(1, 2, figsize=(13, 4.6))
+
+    # Panel 1: forward KL to the ORIGINAL truth
+    ax = axes[0]
+    for arm, (c, lab) in STYLE.items():
+        s = df[df["arm"] == arm].groupby("generation")["forward_kl"]
+        mean, sem = s.mean(), s.sem()
+        ax.plot(mean.index, mean.values, color=c, label=lab)
+        ax.fill_between(mean.index, mean - 1.96 * sem, mean + 1.96 * sem,
+                        color=c, alpha=0.15)
+    for g in remint_gens:
+        ax.axvline(g, ls=":", color="k", lw=0.8, alpha=0.5)
+    ax.set(xlabel="generation", ylabel=r"forward KL to ORIGINAL truth",
+           title="Re-minting while collapsed locks in divergence")
+    ax.legend(frameon=False, fontsize=8)
+
+    # Panel 2: heterozygosity (which arms are collapsed; gate reads this)
+    ax = axes[1]
+    gate = None
+    for arm, (c, lab) in STYLE.items():
+        s = df[df["arm"] == arm].groupby("generation")["heterozygosity"].mean()
+        ax.plot(s.index, s.values, color=c, label=lab)
+    # draw the gate threshold used by the gated arm
+    for v in cfg["sweep"][0]["values"]:
+        if v["name"] == "collapsed_gated":
+            gate = v["set"].get("dynamics.remint.H_gate")
+    if gate is not None:
+        ax.axhline(gate, ls="--", color="k", lw=1)
+        ax.annotate(f"gate H={gate}", (G * 0.02, gate + 0.02), fontsize=8)
+    for g in remint_gens:
+        ax.axvline(g, ls=":", color="k", lw=0.8, alpha=0.5)
+    ax.set(xlabel="generation", ylabel="heterozygosity $H$",
+           title="Diversity at re-mint time (the gate reads this)")
+    ax.legend(frameon=False, fontsize=8)
+
+    fig.suptitle("E6 — re-minting is irreversible; gate it on diversity", y=1.02)
+    fig.tight_layout()
+    savefig(fig, results_dir, "E6")
+
+
+if __name__ == "__main__":
+    main(*sys.argv[1:])