Measure traversal policy boundary cost

Add a microbench that separates game/encoding overhead, single-request policy boundary overhead, and batched PyTorch forward lower bounds. Record CPU/CUDA results and link the finding from the Julia port evaluation.

Co-Authored-By: Codex <codex@openai.com>

experiments/traversal_policy_boundary/README.md

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+# Traversal Policy Boundary Microbench
+
+Purpose: separate the current traversal hot path into game/encoding overhead,
+single-request policy boundary overhead, and batched forward lower bounds.
+
+Run:
+
+```bash
+uv run python experiments/traversal_policy_boundary/bench_policy_boundary.py \
+  --traversals 32 \
+  --runs 5 \
+  --warmup 1 \
+  --corpus-size 512 \
+  --component-repeats 4 \
+  --forward-repeats 32 \
+  --device cpu
+```
+
+CUDA spot check:
+
+```bash
+uv run python experiments/traversal_policy_boundary/bench_policy_boundary.py \
+  --traversals 8 \
+  --runs 3 \
+  --warmup 1 \
+  --corpus-size 512 \
+  --component-repeats 2 \
+  --forward-repeats 32 \
+  --device cuda \
+  --output experiments/traversal_policy_boundary/results_cuda.json
+```
+
+2026-05-07 results, `configs/deep_cfr/default.yaml`, RTX 3090 host:
+
+| Device | Component | Median us/call | p95 us/call |
+| --- | --- | ---: | ---: |
+| CPU | encode + legal | 3.10 | 3.81 |
+| CPU | push + pop | 0.15 | 0.22 |
+| CPU | policy boundary bs=1 | 111.50 | 125.46 |
+| CPU | torch forward bs=64 | 12.84 | 13.14 |
+| CUDA | encode + legal | 3.16 | 3.88 |
+| CUDA | push + pop | 0.16 | 0.25 |
+| CUDA | policy boundary bs=1 | 181.30 | 194.77 |
+| CUDA | torch forward bs=64 | 2.55 | 2.75 |
+
+Interpretation: the game mechanics and state encoding are not the dominant
+cost. The current one-row policy boundary dominates, and CUDA only becomes
+attractive once requests are actually batched.