transforms: pad legacy cond normalizers for pre-physical-conditioning checkpoints

Checkpoints trained before commit 68fb99b (physical-property
conditioning, COND_DIM 8->15) saved a COND_DIM_BASE-wide cond
normalizer, fit before build_cond_features grew the extra physical
columns. Any inference against such a checkpoint under current code
(predict/rollout/router_gating) crashed broadcasting a 15-wide
cond_cont against an 8-wide mean/std.

In "embedding" mode those physical columns are never read by
ConditionEncoder, so padding the missing entries with mean=0/std=1 is
a safe no-op. "physical" mode reads them directly, so a mismatch there
still raises instead of silently normalizing garbage.

tests/test_transforms.py

@@ -397,3 +397,46 @@ def test_build_cond_features_mass_charge_override(fake_material_props):
         cond_cont[:, COND_DIM_BASE], log_transform(np.array([123.0, 456.0]))
     )
     np.testing.assert_allclose(cond_cont[:, COND_DIM_BASE + 1], [2.0, -2.0])
+
+
+def test_build_cond_features_pads_legacy_normalizer_in_embedding_mode():
+    """A pre-physical-conditioning checkpoint's cond normalizer is COND_DIM_BASE
+    (8) wide, fit before build_cond_features grew the extra physical columns.
+    In "embedding" mode those columns are never read downstream, so a legacy
+    normalizer should be usable as-is (padded, not rejected)."""
+    data = _minimal_step_data(3)
+    pdg_map, mat_map = {11: 0}, {"PbWO4": 0}
+    legacy_norm = Normalizer()
+    legacy_norm.mean = np.zeros(COND_DIM_BASE, dtype=np.float32)
+    legacy_norm.std = np.ones(COND_DIM_BASE, dtype=np.float32)
+
+    cond_cont, _ = build_cond_features(
+        data, pdg_map, mat_map, cond_normalizer=legacy_norm, conditioning="embedding"
+    )
+
+    assert cond_cont.shape[-1] == COND_DIM
+    # padded physical columns are zero-filled pre-normalization and
+    # mean=0/std=1 post-normalization, so they should come out as zero
+    np.testing.assert_allclose(cond_cont[:, COND_DIM_BASE:], 0.0)
+
+
+def test_build_cond_features_rejects_legacy_normalizer_in_physical_mode(
+    fake_material_props,
+):
+    """Unlike "embedding" mode, "physical" mode actually reads the physical
+    columns, so a legacy 8-wide normalizer can't be silently padded — that
+    would silently feed the network un-normalized physical properties."""
+    data = _minimal_step_data(3)
+    pdg_map, mat_map = {11: 0}, {"PbWO4": 0}
+    legacy_norm = Normalizer()
+    legacy_norm.mean = np.zeros(COND_DIM_BASE, dtype=np.float32)
+    legacy_norm.std = np.ones(COND_DIM_BASE, dtype=np.float32)
+
+    with pytest.raises(ValueError, match="predates physical-property conditioning"):
+        build_cond_features(
+            data,
+            pdg_map,
+            mat_map,
+            cond_normalizer=legacy_norm,
+            conditioning="physical",
+        )