Skip empty-slice mean/std in sec phys validation print

The per-dim print loop lacked the empty-array guard already used for
the KL computation right above it and the sec-slot loop further down,
so an all-zero-secondaries validation batch (e.g. early/unstable
training) triggered numpy RuntimeWarnings from .mean()/.std() on
empty arrays.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

giant/validate.py

@@ -215,6 +215,8 @@ def validate_marginals(
     print("-" * 68)
     for j, name in enumerate(_SEC_PHYS_NAMES):
         r, g = phys_real[:, j], phys_gen[:, j]
+        if len(r) == 0 or len(g) == 0:
+            continue
         print(
             f"{name:<20} {r.mean():>10.4f} {g.mean():>10.4f} "
             f"{r.std():>10.4f} {g.std():>10.4f} {phys_kl[j]:>14.4f}"