frank-archon: the slab is a threshold, and decode is where slots cost

Measured, 512 tokens, greedy, cold, at the deployed 3 x 200,000 fp8 shape:
prefill 22,208 -> 3,397, 24,000 -> 3,290, 25,600 -> 3,290 t/s at 43.5k —
the third slab step bought nothing, so the slab is a threshold whose cliff is
the 2026-10-11 arm at 26,071 (4,379), not a slope. Decode ladder: 99.0 t/s
single-stream at 43.5k, 93.3 at 197k (nearly window-free), and 3 concurrent
streams 113.9/43.3/10.9 = 18.4 t/s aggregate — the slot count is paid in
decode concurrency, the axis the shape hunt never measured because probe leg
B generates 24 tokens. probe-frank-archon-kv.py gains leg D (N-stream decode,
512 tokens, per-stream + aggregate) so no resize is priced on prefill alone.

serverAppConfigs-docs/log.md

@@ -72,6 +72,26 @@
   raid6 array itself is healthy, 7/7; fstab has `nofail` so boot continued) —
   media libraries on it read empty. `frank-job-digest.service` (frankbook
   job-hunt digest) is in `list-units --state=failed`.
+- **The slab is a threshold, not a slope** — measured, same method (probe A
+  leg: 43,512 tokens, greedy, cold, disk prefix cache clean on every boot):
+  22,208 → 3,397 t/s, 24,000 → 3,290, **25,600 → 3,290**. The third step
+  bought nothing; the +22 % belongs to the 2026-10-11 arm at slab 26,071
+  (4,379 t/s), so the cliff is real and sits at ~26 GiB. The 2-slot arm that
+  would clear it (pool 3,072 + slab 27,897) remains untested — and the
+  paragraph above is amended: that arm's payoff is +22 % prefill at 2 slots,
+  not the +8.9 % the deployed pair ever showed (24,000 → 25,600 = 0 %).
+- **Frank's decode challenge, answered with numbers**: 512 tokens, greedy,
+  cold — **decode 99.0 t/s single-stream at 43.5k and 93.3 at 197k** (decode
+  is nearly window-free: −6 % for 4.5× depth), while **3 concurrent streams
+  give 113.9/43.3/10.9 t/s = 18.4 t/s aggregate** at 43.5k and 12.5 at 197k.
+  The slot count is paid in decode concurrency — a fused-step lottery, not a
+  fair share — which is exactly the axis the shape hunt was blind to (probe
+  leg B generates 24 tokens). Corpus caveat at the claim: the ladder's filler
+  is repetitive, acceptance 0.93–0.97 vs the 0.740 of the 2026-10-11 decode
+  table, so these rows are the baseline for this corpus, not a replacement.
+- **Seam:** `probe-frank-archon-kv.py` gains **leg D** (N-stream decode, 512
+  tokens, per-stream + aggregate) so no future shape change can be priced on
+  prefill alone.
 
 
 ## 2026-10-10