docs(quant): consolidate quantization lessons into a durable playbook

Quants are hard-fought and we keep re-paying for the same lessons. A survey
found quant knowledge scattered across 18 files in four trees, with three
documents having independently discovered and recorded overlapping
"landmines" sections — and one of them now actively misleading.

Adds docs/pfi/model-quantization-playbook.md as the single home for the
TRANSFERABLE lessons, with per-model artifacts demoted to worked examples
that link up to it. Contents:

- scheme decision table, incl. that a literal "W4A8" NVFP4 checkpoint is
  unservable on vLLM (two legal activation settings, FP8 is not one)
- the reference mixed-precision recipe and the three parts of it that are
  load-bearing and easy to drop
- the recurring landmines, ordered by cost: the loader-class trap
  (rediscovered THREE times), the three separate ways to lose the MTP head,
  toolchain deadlocks, vision configs, memory/device placement
- pipeline shape: prove targets before spending GPU time; mandatory
  post-steps that verify rather than assume
- the acceptance gate, and the three ways measurement has lied to us —
  prefix caching faking both speed metrics, prompt_logprobs going uniform
  under speculative decoding, and a 0600 .env making compose silently no-op
- hardware/co-residency, including that a SMALLER model can starve its
  neighbour because gpu-memory-utilization is a fraction of the whole card
- a superseded-claims table, and measured negatives not to re-chase

The superseded table earns its place immediately: the heretic2 runbook tells
readers to use modelopt because "compressed-tensors can't load the BF16 MTP
head, 0% acceptance". That symptom was real but the cause was not the format
-- it was the missing re:^mtp.* ignore entry. compressed-tensors gives
47.7-83.2% acceptance in production. A fresh session following that doc would
be sent down the modelopt path that current memory calls dependency hell, so
the runbook now carries a stale-warning header pointing here.

Wires discovery: an orientation.md "Where to look for what" row, pointers
from the gen-seat / heretic2 / mistral artifacts, and a CLAUDE.md maintenance
rule so the playbook gets fed instead of going stale -- model-agnostic
lessons land in the playbook, model-specific ones stay put, and a wrong
claim earns a dated superseded row rather than a silent edit.

Motivated by Qwen3.8 having just released: the next model swap will need a
requant, and this is what that session should read first.

docs/runbooks/heretic2-nvfp4-mtp-seat.md

@@ -1,5 +1,16 @@
 # Heretic2 NVFP4 + MTP fast char-rp-reasoning seat — the working recipe
 
+> ⚠️ **PARTIALLY SUPERSEDED (2026-08-15). Read [`docs/pfi/model-quantization-playbook.md`](../pfi/model-quantization-playbook.md) first.**
+>
+> Specifically, **landmine 2 below is now false.** "compressed-tensors can't load the BF16 MTP
+> head → 0% acceptance" was a real symptom with the wrong cause: the head was missing from
+> `quantization_config.ignore`, not failed by the format. compressed-tensors + `re:^mtp.*` in
+> ignore gives 47.7–83.2% acceptance in production. **Use compressed-tensors / llm-compressor;
+> do not start a new quant on modelopt** (see the playbook §3.4 and §7).
+>
+> The rest — the loader-class trap, the GPU window ritual, the acceptance-verification method —
+> still holds and is generalized in the playbook.
+
 **Status: WORKING (2026-07-14).** ~77 tok/s single-stream (vs GGUF NEO-CODE ~59.5, base
 NVFP4 ~53) — **~1.3× over GGUF**, MTP draft-acceptance **32–40%**, mean acceptance length
 **2.19**. This is a drop-in faster replacement for the GGUF NEO-CODE `char-rp-reasoning`