epistasis_predicts: functional conflict, not weight geometry, predicts merge failure pre-merge

The decisive experiment from the external review. 39 LoRA parent pairs
(0.5B, 3 seeds) on three axes decorrelated by construction: conflict
(contradictory conventions on shared prompts, private budgets fixed),
compat (same prompts, SAME convention — overlap without conflict), and
duration (weight divergence, zero conflict). Six pre-merge predictors;
primary outcome = merge penalty (parent potential − merged achieved).

League table (Spearman vs penalty, n=39): functional measures predict
(dis_raw +0.460, epi_conf +0.446, p<0.005); geometry collapses
(delta_cos +0.03, delta_l2 +0.17 n.s.); gradient alignment weak (−0.35);
performance ~0. The first grid's apparent geometry win (+0.60) was an
overlap/volume artifact — the compat control axis (added for exactly
this) exposed and killed it: same overlap and data volume, zero penalty.
Honest riders in the README: confidence weighting does not beat raw
disagreement as a rank predictor (pre-registered internal prediction not
confirmed; it does double the conflict/compat level contrast), and
|rho|~0.45 is bounded by 0.5B merge-outcome noise (7B is the firm-up).

Also: micro-batched gradient accumulation (OOM fix on the shared 16GB
GPU), exact r-space LoRA-delta geometry (brute-force-verified test,
151 green), systemd-run runbook lesson (tmux dies with the SSH session
scope on this box).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BkRLcc18rwT2Lysu6PbG7v

results/llm_epistasis_compat/README.md

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+# The overlap-without-conflict control axis
+
+Companion to `results/llm_epistasis/` (full analysis and league table there). Both children train on
+the SAME ambiguous prompts with the SAME convention (asc/asc) — task overlap and added-data volume
+identical to the conflict axis, with zero conflict. Result: merge penalty ≈ 0.005, geometry
+predictors' apparent power collapses (delta_cos ρ +0.60 → +0.03 once these pairs enter the pool),
+functional measures correctly stay low. The control that separated "detects incompatibility" from
+"detects overlap".

results/llm_epistasis_compat/manifest.json

@@ -0,0 +1,26 @@
+{
+  "experiment": "llm_epistasis_compat",
+  "master_seed": 1,
+  "git_commit": "5a23ddaf2a906a14d9aeb2797cf8fef821a519f4",
+  "python": "3.14.7",
+  "libraries": {
+    "numpy": "2.5.0",
+    "scipy": "1.18.0",
+    "pandas": "3.0.3",
+    "pyarrow": "24.0.0",
+    "torch": "2.12.1",
+    "transformers": "5.13.0",
+    "peft": "0.19.1"
+  },
+  "rows": 12,
+  "results_sha256": "e2091aebfdfdeaa1d44c3ca8b9a60d2dfd13bfe546d811f19ce8eb1ccfc14e7b",
+  "layer": "2",
+  "tier": "llm",
+  "base_model": "Qwen/Qwen2.5-0.5B-Instruct",
+  "hard": false,
+  "seeds": [
+    1,
+    2,
+    3
+  ]
+}

results/llm_epistasis_compat/resolved_config.yaml

@@ -0,0 +1,30 @@
+experiment: llm_epistasis_compat
+seed: 1
+n_replicates: 1
+source_config:
+  experiment: llm_epistasis_compat
+  kind: llm_epistasis
+  seed: 1
+  seeds:
+  - 1
+  - 2
+  - 3
+  n_replicates: 1
+  base_model: Qwen/Qwen2.5-0.5B-Instruct
+  family_a: strings
+  family_b: arith
+  n_train: 400
+  n_test: 80
+  epochs: 3
+  n_probe_each: 30
+  grad_k: 32
+  lora:
+    r: 16
+    alpha: 32
+  compat_fracs:
+  - 0.25
+  - 0.5
+  - 0.75
+  - 1.0
+  output:
+    dir: results/llm_epistasis_compat