feat(hpo): env-var knob overrides + sweep script (18 configs)

- train.py knobs all readable from JEPA_* env vars (JEPA_WINDOW, JEPA_D_MODEL,
  JEPA_DEPTH, etc.) so hpo_sweep.py can override without touching source
- scripts/hpo_sweep.py: 3×2×3 grid over D_MODEL × DEPTH × WINDOW,
  logs to results/hpo/hpo_results.jsonl with leaderboard at end
- 3 new tests: env override correctness, configs() schema validation
- 19/19 tests pass

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_hepa.py

@@ -1,9 +1,10 @@
-"""Failing tests for HEPA backbone + Phase-1 supervised head in train.py.
+"""Failing tests for HEPA backbone + Phase-1 supervised head + HPO in train.py.
 
 Run: cd ~/dev/AI/jepa-fx-risk && .venv/bin/python -m pytest tests/test_hepa.py -v
 These tests define what the backbone and head must satisfy BEFORE implementation.
 """
 import math
+import os
 import torch
 import torch.nn as nn
 import pytest
@@ -12,11 +13,24 @@ import pytest
 # They will fail until train.py implements: CausalEncoder, HorizonPredictor, vicreg_loss
 
 
-def _import():
+def _import(env_overrides=None):
     import importlib.util, sys
-    spec = importlib.util.spec_from_file_location("train", "train.py")
+    saved = {}
+    if env_overrides:
+        for k, v in env_overrides.items():
+            saved[k] = os.environ.get(k)
+            os.environ[k] = str(v)
+    # Force fresh module load (env vars must be read at import time)
+    name = f"train_{id(env_overrides)}"
+    spec = importlib.util.spec_from_file_location(name, "train.py")
     mod = importlib.util.module_from_spec(spec)
     spec.loader.exec_module(mod)
+    if env_overrides:
+        for k, orig in saved.items():
+            if orig is None:
+                os.environ.pop(k, None)
+            else:
+                os.environ[k] = orig
     return mod
 
 
@@ -171,7 +185,7 @@ def test_phase1_beats_linear_on_nonlinear(train_mod):
 
 # 11. main() returns phase1_r2 in metrics.json (integration — needs real data)
 def test_metrics_json_has_phase1_r2(train_mod):
-    import os, json
+    import json
     if not os.path.exists("metrics.json"):
         pytest.skip("metrics.json not present — run train.py first")
     with open("metrics.json") as f:
@@ -181,3 +195,35 @@ def test_metrics_json_has_phase1_r2(train_mod):
         f"MLP head phase1_r2={m['phase1_r2']:.4f} should beat linear probe "
         f"val_vol_r2={m['val_vol_r2']:.4f}"
     )
+
+
+# ── HPO: env-var knob overrides ───────────────────────────────────────────────
+
+# 12. JEPA_WINDOW env var overrides WINDOW at import time
+def test_env_override_window():
+    mod = _import({"JEPA_WINDOW": "48"})
+    assert mod.WINDOW == 48, f"expected WINDOW=48, got {mod.WINDOW}"
+
+
+# 13. JEPA_D_MODEL and JEPA_DEPTH env vars work
+def test_env_override_d_model_depth():
+    mod = _import({"JEPA_D_MODEL": "64", "JEPA_DEPTH": "4"})
+    assert mod.D_MODEL == 64, f"expected D_MODEL=64, got {mod.D_MODEL}"
+    assert mod.DEPTH == 4, f"expected DEPTH=4, got {mod.DEPTH}"
+
+
+# 14. hpo_sweep.py exists and generates correct config list
+def test_hpo_sweep_configs():
+    import importlib.util
+    sweep_path = "scripts/hpo_sweep.py"
+    if not os.path.exists(sweep_path):
+        pytest.fail(f"{sweep_path} not found — implement it")
+    spec = importlib.util.spec_from_file_location("hpo_sweep", sweep_path)
+    mod = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(mod)
+    cfgs = list(mod.configs())
+    assert len(cfgs) > 0, "configs() returned empty list"
+    # Every config must have at least D_MODEL, DEPTH, WINDOW keys
+    required = {"JEPA_D_MODEL", "JEPA_DEPTH", "JEPA_WINDOW"}
+    for cfg in cfgs:
+        assert required.issubset(cfg.keys()), f"config missing required keys: {cfg}"