f2c: switch to per-instance decode-concurrency view; correct KV pool ceiling

Old f2c plotted per-request KV footprint MiB against an "H20 ~95 GiB
usable" reference line. That ceiling was wrong — a 30B-A3B bf16
deployment burns roughly:
  ~50% HBM for model params (~48 GiB on 96 GiB H20)
  ~10% for runtime activation buffers
  ~40% left for the KV cache pool (~38.4 GiB)
so 95 GiB was overstating the available pool by 2.5×.

New f2c reframes the same data into the answer that actually motivates
the paper: how many concurrent decodes does a single instance hold,
and how does PD-disagg change that? Grouped bars per percentile show
system-wide concurrent decode capacity for three 8-GPU deployments:
  Combined 8C, PD-disagg 4P+4D (N_D=4), PD-disagg 6P+2D (N_D=2)

Key reads off the figure:
  p50 (1.8 GiB/req): 20 fit/inst → 160 / 80 / 40 system-wide
  p90 (8.0 GiB/req):  4 fit/inst →  32 / 16 /  8
  p95 (9.6 GiB/req):  4 fit/inst →  32 / 16 /  8
  p99 (11.5 GiB/req): 3 fit/inst →  24 / 12 /  6

PD-disagg 4P+4D literally halves the decode population at the same
per-request KV pressure — this is the concrete §3.2 "KV memory wall"
penalty stated in terms users care about (concurrency).

- analysis/characterization/render_window1_figures.py:
  fig_kv_footprint_cdf rewritten; reads same kv_footprint_summary.json
  but computes floor(KV_pool / req_size) × N_D and annotates the
  per-instance fit count below each percentile group.
- figs/f2c_kv_footprint_cdf.png: regenerated.
- MEETING.md / PAPER_OUTLINE.md §2.1, §2.4: prose updated with the
  new ceiling and the "3 p99 decodes per instance / halved by PD-disagg"
  framing.

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

MEETING.md

@@ -27,7 +27,7 @@ L = Λ · N · W_turn(L)        # agentic, T_human≈0
 |---|---|---|
 | KV reuse 几乎只在 session 内 | intra 93.2% / cross 5.7% / shared 1.1% | ![](figs/f2a_reuse_topology.png) |
 | Session 极度偏斜 | production trace 上 top 1% / 5% / 10% / 25% / 50% = **46.5% / 66.5% / 74.6% / 87.5% / 96.0%** input mass | ![](figs/f2b_session_skew.png) |
-| 单请求 KV footprint 已经很大 | p99 = 11.8 GiB ≈ H20 12% | ![](figs/f2c_kv_footprint_cdf.png) |
+| 单请求 KV footprint 大，单 instance KV pool 很快被占满 | per-instance KV pool ≈ **38 GiB**（0.4 × 96 GiB H20，剩 50% params + 10% activation）；p99 req 11.5 GiB → 一个 instance 只装 **3 个 p99 decode**；4P+4D 让系统 decode 容量直接减半 | ![](figs/f2c_kv_footprint_cdf.png) |
 
 理论 APC 上界 = intra-session 79.6% / any-session 80.3%，差 <1pp。**任何不 affinity 的调度都丢绝大部分 reuse。**