revert(gen-seat): back to MTP-off known-good — APC-off mitigation failed in real use

The MTP-on + prefix-caching-off mitigation (63a3cb2) passed synthetic
7-turn probes but the operator still saw severe degeneration in real use.
A passing synthetic probe is NOT sufficient evidence -- it under-covers
real workloads (content distribution, conversation depth). Reverted to the
verified known-good: MTP off, prefix caching on (the 7bd38b3 state), ~half
decode speed but coherent. Operator is driving it to re-confirm.

Lesson reinforced (the recurring one this session): do not trust a
synthetic reproduction to VALIDATE a fix for a bug that only manifests in
the operator's real usage -- it validated a non-fix twice tonight.

stacks/gen-seat/compose.yaml

@@ -56,14 +56,11 @@ services:
       - float32
       - --kv-cache-dtype
       - ${GEN_KV_CACHE_DTYPE:-fp8}
-      # ⚠️ PREFIX CACHING DISABLED 2026-08-17 as the MTP mitigation. vLLM V1
-      # defaults prefix caching ON, so the explicit --no- form is required to
-      # turn it off. With MTP re-enabled below, APC-off forces the GDN cache
-      # into a mode where the partial-accept align-path bug is inert
-      # (vllm#43559 / #47194): the corruption is gated on MTP × prefix-caching
-      # together. Trade vs the alternative (MTP off): this keeps MTP's ~2x decode
-      # and loses only prefix-cache reuse. GATED ON the multi-turn probe passing.
-      - --no-enable-prefix-caching
+      # Prefix caching ON (vLLM V1 default). The APC-off MTP mitigation was tried
+      # 2026-08-17 and did NOT hold — synthetic 7-turn probes passed but the
+      # operator still saw severe degeneration in real use, so it was reverted.
+      # See the MTP block below.
+      - --enable-prefix-caching
       - --enable-chunked-prefill
       - --limit-mm-per-prompt
       - '{"image": 4}'
@@ -102,7 +99,14 @@ services:
       - --enable-auto-tool-choice
       - --tool-call-parser
       - qwen3_coder
-      # MTP RE-ENABLED 2026-08-17, PAIRED WITH --no-enable-prefix-caching above.
+      # ⚠️ MTP / SPECULATIVE DECODING DISABLED — the verified known-good.
+      # Re-enable ONLY after real-use multi-turn testing, not a synthetic probe.
+      #
+      # History: MTP-off is clean. The APC-off mitigation (keep MTP, disable
+      # prefix caching) passed synthetic 7-turn probes on 2026-08-17 but the
+      # operator STILL saw severe degeneration in real use — so a passing probe
+      # is NOT sufficient evidence; reverted to MTP-off. The synthetic probes
+      # structurally under-cover real workloads (content distribution, depth).
       #
       # ROOT CAUSE (isolated 2026-08-16, operator-confirmed): `qwen3_5_mtp`
       # speculative decoding CORRUPTS Qwen3.8-27B output in multi-turn
@@ -116,20 +120,16 @@ services:
       # NOT depth-dependent (n=1 and n=2 both corrupt, so num_speculative_tokens=1
       # is NOT a fix).
       #
-      # THE MITIGATION (this config): the corruption is gated on MTP × prefix-
-      # caching TOGETHER (#43559/#47194). Disabling prefix caching forces the GDN
-      # cache into a mode where the buggy align-path is inert, so MTP can stay on.
-      # Keeps MTP's ~2x decode; costs only prefix-cache reuse. The proper upstream
-      # fix (#51113) is in vLLM main / v0.27.2rc0 only — not in a stable release
-      # as of this date, so we do not jump the fleet gateway to an RC.
+      # The APC-off mitigation (#43559/#47194) was supposed to make the GDN
+      # align-path bug inert while keeping MTP. It did not survive real use.
+      # The proper upstream fix (#51113) is in vLLM main / v0.27.2rc0 only — not
+      # 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.
       #
-      # ⚠️ GATED ON THE MULTI-TURN PROBE. If the 7-turn varied series
-      # (services/gen-seat-mixed-quant/bench/) still collapses past ~2k tokens
-      # with this config, APC-off did not hold on our stack — revert to MTP OFF
-      # (comment these two lines, restore --enable-prefix-caching) which is the
-      # verified known-good.
-      - --speculative-config
-      - '{"method": "${GEN_SPEC_METHOD:-qwen3_5_mtp}", "num_speculative_tokens": ${GEN_SPEC_TOKENS:-3}}'
+      # TO RE-ENABLE (only with real-use verification): uncomment the two lines
+      # below. Do NOT trust a synthetic probe alone — it passed and was wrong.
+      # - --speculative-config
+      # - '{"method": "${GEN_SPEC_METHOD:-qwen3_5_mtp}", "num_speculative_tokens": ${GEN_SPEC_TOKENS:-3}}'
     deploy:
       resources:
         reservations: