feat: isolate DeepSeek history boundary token

experiments/deepseek/README.md

@@ -319,3 +319,55 @@ in 24/24 layer×domain cells, with all 24 paired intervals below zero. See
 `research/DEEPSEEK_ROUTING_HISTORY_DISTANCE_CONTROL_AUDIT.md` for the full
 table, token-level route stability, BF16 batch-shape boundary, literature
 context, and non-claims.
+
+## History-boundary single-token control
+
+`v2_lite_routing_history_boundary_token_control.py` keeps the repeated-token
+history from the preceding probe and changes exactly one token ID at the
+completed assistant boundary:
+
+```text
+system off/on × official EOS / x / period / newline
+```
+
+The official template places EOS between the filler assistant content and the
+next `User:` marker. The three controls replace only that EOS ID after official
+tokenization. They are explicit counterfactual token sequences, not valid
+official chat serializations. All eight conditions preserve sequence length,
+target position, role markers, attention mask, and within-run batch shape.
+
+```bash
+PYTHONPATH=/path/to/transformers-4.41.2-deps:/usr/lib/python3/dist-packages \
+python -B experiments/deepseek/v2_lite_routing_history_boundary_token_control.py \
+  --artifact-dir /path/to/deepseek-v2-lite \
+  --human-eval /path/to/HumanEval.jsonl.gz \
+  --gsm8k /path/to/gsm8k/test.jsonl \
+  --tnews /path/to/tnews/test.json \
+  --tnews-archive /path/to/tnews_public.zip \
+  --wikitext /path/to/wikitext-validation.parquet \
+  --output src/data/deepseek-v2-lite-routing-history-boundary-token-control.json \
+  --per-domain 32 \
+  --content-tokens 23 \
+  --batch-prompts 4 \
+  --layers 7 \
+  --bootstrap 2000 \
+  --seed 20260729 \
+  --captured-at 2026-07-29T11:12:00+00:00
+```
+
+The eight cells add 2,044,224 real top-6 route selections. All 256
+source×system groups are equal-length and equal-position; all 768
+counterfactual cells differ from their official sequence at exactly one input
+ID. The committed run and independent rerun are byte-exact:
+
+```text
+9bb93834ffd8536aeebe4325e45d2179ba590554c6ff6b6fcceaba2f499b9c37
+```
+
+For exact target content under prompt-balanced aggregation, mean system-edge
+TV is `.0374 / .0539 / .0553 / .0492` for EOS / x / period / newline. X and
+period exceed EOS in 24/24 layer×domain cells, while newline does so in 23/24.
+See `research/DEEPSEEK_ROUTING_HISTORY_BOUNDARY_TOKEN_AUDIT.md` for all paired
+intervals, per-token route alignment, scope split, BF16 batch-shape audit,
+primary literature, and the boundary between an input-ID intervention and a
+chat-turn semantic claim.