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- a superseded-claims table, and measured negatives not to re-chase

The superseded table earns its place immediately: the heretic2 runbook tells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claim earns a dated superseded row rather than a silent edit.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.

tools/mistral-small4-nvfp4/README.md

@@ -1,5 +1,9 @@
 # Mistral Small 4 → NVFP4 (vision-intact) build tooling
 
+> General quant lessons are consolidated in
+> [`docs/pfi/model-quantization-playbook.md`](../../docs/pfi/model-quantization-playbook.md);
+> the memory/device gotchas below are generalized there as §3.6.
+
 Quantize a **HF-format** `Mistral3ForConditionalGeneration` checkpoint
 (Mistral Small 4, 119B-total / 6.5B-active MoE) to **NVFP4** with the vision
 tower intact, then convert it to **Mistral native format** so vLLM can serve it.