fix(gen-seat): AEON W4A4 was the defect — purged; mixed FP8-attn build is primary gen

Root cause of the multi-day degeneration hunt, operator-confirmed: the AEON
NVFP4 W4A4 quant (sakamakismile/Qwen3.8-27B-AEON-ULTIMATE-UNCENSORED-NVFP4,
full W4A4 incl. attention) went degenerate ~15-20% of generations in real
multi-turn use and forced regenerates. MTP, prefix-caching, and the gateway
all merely AMPLIFIED it, which is why MTP-off, APC-off, and the vLLM #51113
fix each 'helped' a synthetic probe without fixing it -- three plausible
false root-causes, each passing one clean run then failing in real use.

The fix was the WEIGHTS: the in-house JonathanColetti/Heretic mixed
NVFP4+FP8 build (qwen38-27b-uncensored-nvfp4-mixed, FP8 attention not W4A4,
same base, same MTP) is coherent through long multi-turn with MTP ON. W4A4
*attention* was the defect; FP8 attention is not.

This commit:
- GEN_MODEL -> the mixed FP8-attn build (primary gen until DavidAU 3.8 lands)
- GEN_IMAGE pinned to vllm/vllm-openai:nightly-311b3513... (v0.27.2rc1.dev150,
  carries #51113; pinned by sha so it does not drift on the next pull)
- AEON weights PURGED from /tank (no-good), safety-checked not-in-use first
- playbook 3.8: the stochastic-W4A4-degeneration lesson + isolate-weights-early
  + do-not-declare-a-fix-from-one-probe (it validated three non-fixes)

AEON is re-pullable from HF if ever needed, but the operator ruled it no-good.

stacks/gen-seat/.env.example

@@ -1,5 +1,5 @@
 # gen-seat tunables — fleet `gen` seat (ana-ml2 GPU 0, :8015). Edit here, never commit.
-GEN_IMAGE=vllm/vllm-openai:latest
+GEN_IMAGE=vllm/vllm-openai:nightly-311b3513af33bc29b4acb2fde2e9313e5e9966a0
 API_KEY=
 GEN_GPU_ID=0
 

stacks/gen-seat/compose.yaml

@@ -126,10 +126,18 @@ services:
       # in a stable release — so we do not jump the fleet gateway to an RC.
       # Cost of MTP-off: ~half decode tok/s. Accepted as the known-good.
       #
-      # TO RE-ENABLE (only with real-use verification): uncomment the two lines
-      # below. Do NOT trust a synthetic probe alone — it passed and was wrong.
-      # - --speculative-config
-      # - '{"method": "${GEN_SPEC_METHOD:-qwen3_5_mtp}", "num_speculative_tokens": ${GEN_SPEC_TOKENS:-3}}'
+      # MTP ON — PRIMARY GEN CONFIG (resolved 2026-08-17, operator-confirmed
+      # coherent). The whole multi-day degeneration hunt root-caused to the AEON
+      # W4A4 quant being DEFECTIVE: it went degenerate ~15-20% of generations and
+      # forced a regenerate. MTP / prefix-caching / gateway all merely AMPLIFIED
+      # it, which is why MTP-off and the vLLM #51113 fix each "helped" without
+      # fixing it. This build — the in-house JonathanColetti/Heretic mixed
+      # NVFP4+FP8 weights (FP8 attention, NOT W4A4) on vLLM nightly (#51113 fix)
+      # with MTP ON — is coherent through long multi-turn in real use. AEON
+      # (qwen38-27b-aeon-ultimate-nvfp4) was PURGED as no-good. Primary gen until
+      # the DavidAU Qwen3.8 lands.
+      - --speculative-config
+      - '{"method": "${GEN_SPEC_METHOD:-qwen3_5_mtp}", "num_speculative_tokens": ${GEN_SPEC_TOKENS:-3}}'
     deploy:
       resources:
         reservations: