Restore the privileged critic: its ablation flips with scale

The 39.3M ablation said the privileged critic hurt (switching it off won 0.5160
[0.505, 0.527]). Head to head at 131M, both sides trained identically, it says the
opposite: off *loses*, 0.4633 [0.453, 0.474]. Against league the critic-on model
scores 0.6094 and the critic-off one 0.5526.

The critic earns its keep once there is enough data to fit it -- at 39.3M the
privileged value trunk is underfit and only adds advantage noise. Defaulted back
on, with the small-scale number kept in the docstring as a warning: an ablation at
a budget you do not intend to ship can invert.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XBQKgvBbxbheiTF1AVy1Sh

tests/lost_cities_jax/test_match_ppo.py

@@ -13,6 +13,7 @@ from lost_cities_jax.match_obs import (
     match_observation,
 )
 from lost_cities_jax.match_ppo import (
+    Ablation,
     create_match_train_state,
     make_match_rollout_fn,
     make_match_train_iteration,
@@ -54,9 +55,14 @@ def test_the_critic_sees_the_opponents_hand_and_the_actor_does_not():
 
 
 def test_privileged_input_cannot_move_the_policy_logits():
-    """Separate trunks, or the critic's view of the deck leaks into play."""
+    """Separate trunks, or the critic's view of the deck leaks into play.
+
+    The privileged critic is off by default -- it measured negative, see the
+    ablation in the plan -- but the isolation property still has to hold for
+    anyone who switches it on, and for its later reuse as a search evaluator.
+    """
     cfg = _cfg()
-    state = create_match_train_state(cfg, jax.random.PRNGKey(1))
+    state = create_match_train_state(cfg, jax.random.PRNGKey(1), Ablation(privileged_critic=True))
     match = match_reset(jax.random.PRNGKey(2))
 
     obs = match_observation(match, jnp.int32(0))[None, :]