Add gravity-only attribution for rotating higher-index gain

experiments/COUPLED_CONVERGENCE_STUDY_2026-09-19.md

@@ -220,6 +220,16 @@ fresh baseline. No additional raw trial grid is needed for this attribution.
 The isolated branch is `research/n3-linear-ablation-20260919`, with case
 manifest `convergence_linear_followup_cases_2026-09-19.json`.
 
+The completed n=1.5,J=.1 frozen pair subsequently gives the first clear
+combined-candidate total-cost improvement: 246 to 95 iterations and 117.925
+to 101.760 setup/solve/check seconds, with both eta=.003 gates passing and
+the same largest sampled acceptable alpha=1/16. Add its matched gravity-only
+control to the same follow-up manifest before attributing this 13.71% time
+reduction to the new surface block. This two-case attribution hypothesis is
+isolated on `research/gravity-coupling-ablation-20260919`; the original n=3
+specification branch is retained. These controls add no new nonlinear runs
+or trial grids.
+
 ## Completed matched gravity-path test
 
 The retained-artifact audit passes both six-update restarts. Both begin at

experiments/convergence_linear_followup_cases_2026-09-19.json

@@ -29,5 +29,37 @@
       "../../diagnostics/coupled_convergence_2026-09-19/screen_v1/n3_J0_control/capture_7/physical_base.txt",
       "--allow-unconverged-restart"
     ]
+  },
+  {
+    "id": "n1p5_rotating_late_gravity",
+    "arguments": [
+      "--index",
+      "1.5",
+      "--angular-momentum",
+      "0.1",
+      "--surface-prescription",
+      "harmonics",
+      "--surface-degree",
+      "4",
+      "--restart",
+      "160",
+      "--max-linear",
+      "300",
+      "--rtol",
+      "0",
+      "--atol",
+      "1e-12",
+      "--max-newton",
+      "0",
+      "--linear-tol",
+      ".003",
+      "--preconditioner-probe",
+      "--preconditioner-variant",
+      "gravity-schur",
+      "--restart-snapshot",
+      "../../diagnostics/coupled_convergence_2026-09-19/screen_v1/n1p5_J0p1/accepted_state.txt",
+      "--allow-unconverged-restart",
+      "--no-central"
+    ]
   }
 ]