PD_DISAGG_RESULTS §6.3: producer hot-pinning figure

Direct per-producer KV-pool evidence for the session-affinity backfire.
At the same 4P+4D ratio:
- round-robin: 4 producers within 1pp of each other (spread 0pp, CV 0.01)
- session-affinity: spread 49pp (one producer ~93%, another 45%; CV 0.25)

A 25x jump in producer load imbalance — heavy multi-turn sessions
concentrate onto single producers, the same hot-pinning pathology as
sticky routing in the colocated §3.3 study.

plot_producer_hotspot.py: reduce (numpy, per-producer KV timeline from
snapshots, runs on the serving host) + plot (matplotlib, 2-panel rr vs
session comparison) — same two-stage pattern as aggregate_mb5.py.

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

microbench/fresh_setup/PD_DISAGG_RESULTS.md

@@ -274,6 +274,14 @@ failed transfer also pins the producer's KV (it is not freed on
 `kv_load_failure_policy=fail`), compounding the stall until the pipeline
 deadlocks at ~0% utilization.
 
+The per-producer KV-pool timelines make the hot-pinning direct. At the **same
+4P+4D ratio**, round-robin holds all four producers within **1 percentage
+point** of each other (spread 0pp, CV 0.01); session-affinity blows the spread
+open to **49 percentage points** (one producer pegged at ~93% while another
+sits at 45%, CV 0.25 — a 25× jump in load imbalance):
+
+![per-producer KV pool: round-robin vs session-affinity](../../figs/mb5/mb5_producer_hotspot.png)
+
 Producer-side prefix-cache hit in the degraded state is ~0.2% (vs round-robin's
 ~5%) — session-affinity never even gets to *collect* the cache-reuse benefit it
 was supposed to provide, because the producers it concentrates load onto are