feat: add DeepSeek equal-length history control

experiments/deepseek/README.md

@@ -264,3 +264,58 @@ See `research/DEEPSEEK_ROUTING_HISTORY_FACTORIAL_AUDIT.md` for the factorial
 definition, all CV intervals, the 24/24 system-edge TV attenuation pattern,
 per-token stability, and the strict boundary between a message-history effect
 and a role-semantic claim.
+
+## Equal-length filler history control
+
+`v2_lite_routing_history_distance_control.py` extends the preceding 2×2 probe
+to six cells:
+
+```text
+system off/on × no history / repeated-token filler / fixed one-shot
+```
+
+The filler turn is:
+
+```text
+User: x x x x x x x x x
+Assistant: x
+```
+
+Under the pinned official template, both filler and one-shot add exactly 17
+tokens per source on both system levels. They therefore share the same
+user/assistant roles, assistant EOS, target position, generation prompt, and
+batch shape. The filler is deliberately called low-information rather than
+semantics-free: repeated `x` tokens remain learned inputs.
+
+```bash
+PYTHONPATH=/path/to/transformers-4.41.2-deps:/usr/lib/python3/dist-packages \
+python -B experiments/deepseek/v2_lite_routing_history_distance_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-distance-control.json \
+  --per-domain 32 \
+  --content-tokens 23 \
+  --batch-prompts 5 \
+  --layers 7 \
+  --bootstrap 2000 \
+  --seed 20260729 \
+  --captured-at 2026-07-29T10:17:00+00:00
+```
+
+The six cells add 1,376,496 real top-6 route selections. The committed run and
+independent rerun are byte-exact:
+
+```text
+423a095d92738d4bc6c2ace2afa9b35627e61811458c7883dca7b44aee4e648e
+```
+
+For exact target content under prompt-balanced aggregation, mean system-edge
+TV is `0.0738 → 0.0378 → 0.0188` for none → filler → demo. Both steps decrease
+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.