Add JAX PPO static-opponent trainer

README.md

@@ -196,6 +196,47 @@ steps_per_sec=3956598.38
 - Observation scalar normalization is implementation-defined as documented
   above and locked by the exported `OBS_DIM`.
 
+## JAX PPO Static-Opponent Ladder
+
+The first training stack above `lost_cities_jax` is exposed as:
+
+```bash
+uv run lost-cities-jax-ppo --help
+```
+
+CPU smoke:
+
+```bash
+uv run lost-cities-jax-ppo rollout-smoke --config configs/jax_ppo/smoke.yaml
+uv run lost-cities-jax-ppo train --config configs/jax_ppo/smoke.yaml
+```
+
+GPU training uses optional CUDA JAX, keeping CUDA wheels out of the default
+project dependency set:
+
+```bash
+tmux new-session -s coolrl-jax-ppo-discard \
+  -c /home/coolguy/dev/coolrl-lost-cities \
+  "flock -n .compute.lock uv run --with 'jax[cuda12]' lost-cities-jax-ppo train \
+    --config configs/jax_ppo/discard-only.yaml"
+```
+
+Random-policy baseline vs `discard_only`, measured on 2026-07-04 with
+8192 games x 400 plies on GPU:
+
+```text
+return_mean=-55.582763671875
+game_length_mean=69.6085205078125
+max_steps_rate=0.0
+play_action_rate=0.28854578733444214
+opened_colors_mean=4.942626953125
+positive_expeditions_mean=0.41796875
+```
+
+Large PPO artifacts are written under
+`/mnt/2tbhdd/coolrl-lost-cities-artifacts/jax-ppo-static-opponents/`.
+Generated checkpoints and evaluation JSON are not committed to git.
+
 ## Basic Usage
 
 ```python