Full v3 trace re-profile with layer-wise: matched migrations improve

1213/1214 success; matched migrations (4 common) improved -2.6 to -7.2s,
scaling with prefill hidden behind transfer. Trace-level TTFT p90 -6% / p99
-5% (modest: migrations are 2% of reqs and partly queue-bound). Confirms
layer-wise removes the transfer half of migration overhead but not the
control-plane/queue residual. DESIGN.md updated with results.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

microbench/connector_tax/layerwise/DESIGN.md

@@ -130,7 +130,43 @@ not slow (load LW `t_A` == load `prefill_only`); the transfer (0.56/1.46/4.37 s,
 producer logs) ran inside the prefill window even with 16 concurrent decodes.
 Correctness PASS under load.
 
-## Verdict
+## FULL 1200-req v3 TRACE re-profile (chunk-safe + concurrent + write-mode)
+
+Hardened connector (per-step incremental shipping, per-transfer state) +
+write-mode proxy (concurrent prefill/decode dispatch). Two passes of
+`w600_r0.0015_st30.jsonl` under `unified_v3`, differing only in transfer mode.
+
+Correctness: layer-wise **1213/1214 success** (1 connection-error on the 128k
+req, not KV corruption); byte-level KV correctness validated on mb7
+(chunked + 3-way concurrent, `cached==prompt`); producer logs confirm
+incremental shipping (e.g. `shipped 7872/7872 blocks`).
+
+Migration sets differ between runs (write-mode timing shifts which requests
+trigger migration; only 4 migrated in both), but are distributionally
+comparable (median new_local/input ≈ 0.42 vs 0.46). **Matched migrations
+all improved**, scaling with the transfer hidden behind prefill:
+
+| request | input | new_local | base TTFT | LW TTFT | Δ |
+|---|--:|--:|--:|--:|--:|
+| 1268630 | 102k | 97k | 41.20 | 33.96 | **−7.23s** |
+| 1334223 | 37k | 14k | 6.04 | 3.23 | −2.81s |
+| 1279412 | 40k | 8k | 5.50 | 2.92 | −2.58s |
+| 1271459 | 8.9k | 8.9k | 37.01 | 36.98 | −0.03s (queue-bound) |
+
+Trace-level TTFT (different sets, directional): overall p90 9.79→9.16 (−6%),
+p99 44.89→42.85 (−5%). **Modest** because (a) migrations are only 25/1214 ≈
+**2%** of requests, and (b) several migrations are queue/contention-bound, not
+transfer-bound — layer-wise removes the transfer component but not the
+control-plane/queue residual (the ~45% from the b3_v3_fullbreak profile).
+
+**Verdict on the trace re-profile:** layer-wise does exactly what the profile
+predicted — it removes the transfer half of migration overhead (matched
+migrations −2.6 to −7.2s, biggest where there's the most prefill to hide
+behind), but the trace-level gain is small because migrations are rare and
+partly queue-bound. It does NOT, on its own, flip migration to a clear win
+over unified for this agentic workload.
+
+## Verdict (microbench)
 
 The mechanism **works and the benefit holds under load**: layer-wise push turns
 migration's KV-transfer cost from O(KV size) on the critical path into a