docs(quant): record the MTP-corrupts-Qwen3.8-multi-turn lesson (playbook 3.7)

The single hardest bug of the night, and invisible to the existing
acceptance gate: a LOADED, healthy-accepting MTP head still corrupts
Qwen3.8-27B multi-turn output past ~2k cumulative tokens (length collapse +
cross-turn content bleed), while single-turn is perfect. Model-independent
across all three of our Qwen3.8 quants; Qwen3.6 on the same qwen3_5_mtp
method is clean; disabling MTP fixes it. New rule: gate MTP on a multi-turn
coherence probe, not just single-shot acceptance.

docs/pfi/model-quantization-playbook.md

@@ -131,6 +131,39 @@ There is also a **modelopt-format-specific** version of this: vLLM 0.24 does not
 modelopt `exclude_modules` to the spec-decode *draft* model, which no checkpoint config can fix
 (needs a `sitecustomize` runtime patch). Using compressed-tensors avoids it entirely — §3.4.
 
+### 3.7 ⭐ A LOADED MTP head can still corrupt output — Qwen3.8 multi-turn
+
+§3.3 is about *losing* the head (0% acceptance, silent). This is the opposite and
+worse failure: the head loads, acceptance looks healthy, single-turn output is
+perfect — and then it **corrupts multi-turn conversations** once cumulative context
+passes **~2,000 tokens**. The reply collapses in length *and* bleeds earlier turns
+into the current answer (a "describe durian" reply that contained the Krebs-cycle
+and winter answers from three turns back). Single-turn probes and the acceptance
+gate (§5) **do not catch it** — it only appears as accumulated context grows.
+
+Isolated 2026-08-16 (operator-confirmed), each step measured on a fixed 7-turn probe:
+
+- **Not the serving gateway, not sampling, not repetition/template.** Identical
+  input gateway-vs-direct behaves the same; presence_penalty 1.5/0.5/0.0 all
+  collapse; higher temperature collapses harder; a conversation of *unrelated*
+  topics collapses at the same ~2k tokens as a repetitive one → it is context-
+  length-driven, not template lock-in.
+- **Model-independent across every Qwen3.8-27B quant** (AEON W4A4, unsloth
+  FP8-attn, our in-house mixed) — so not a quant-brand or scheme artifact.
+- **DECISIVE: same model + same conversation, MTP OFF → coherent through 4k+
+  tokens, zero bleed.** Toggle it back on → collapse returns. MTP is the cause.
+
+**Qwen3.6-27B running the same `qwen3_5_mtp` method is CLEAN.** So the 3.6 MTP
+head/graft is fine and the 3.8 one is not — suspects: the bf16 graft being subtly
+wrong for the 3.8 head, or the vLLM `qwen3_5_mtp` impl diverging at `num_speculative_tokens=3`.
+Open upstream question (queried dvalin/bil-smithy 2026-08-17).
+
+**Rule: gate MTP on a MULTI-TURN coherence probe, not just single-shot acceptance.**
+Run a 7-turn varied-topic conversation and watch turns past ~2k cumulative tokens
+for length-collapse and cross-turn bleed. If it corrupts: disable MTP (costs ~half
+decode tok/s) or try `num_speculative_tokens=1` (shallowest draft, least divergence
+— untested as of this writing). The gen seat runs MTP **off** as of 2026-08-17.
+
 ### 3.4 Toolchain version deadlocks
 
 Both directions have burned us, so the resolution is: **use llm-compressor / compressed-tensors,