docs(frank-archon): work-log entry and the 4-way capacity probe

Probe on the live service: four concurrent 196,960-token chats, 4/4 exact
marker retrieval, 787,842 tokens in 238.4 s (3,305 t/s aggregate prefill = the
single-stream ceiling, so the four streams shared one pipeline and the KV pool
never ran short); prefix-cache repeat answered in 1.0 s with cache_n=43008.
Recorded with the caveat that 8 generated tokens do not make a decode rate.

serverAppConfigs-docs/LLM/frank-archon-radiance-swift15-w4dn8.md

@@ -385,6 +385,33 @@ Oct 13 19:21:08 dankserver systemd[1]: Started frank-archon.service
 the boot, i.e. the root `ExecStartPre` hook did the write the service process
 (User=dank) cannot.
 
+### Capacity probe (2026-10-13, live service)
+
+`scripts/probe-frank-archon-kv.py` now reads the slot count, window and KV dtype
+from the deployed env on the host, so the probe cannot drift from the service:
+it printed `shape: 4 x 200,000 tokens, kv fp8` and fired **four concurrent chats
+of 196,960 prompt tokens each** (646,299 chars, 98.5 % of the window), each
+carrying a marker whose first word is unique.
+
+| Stream | prompt | wall | prefill | retrieval |
+|---|---|---|---|---|
+| B1 | 196,961 | 238.3 s | 1,423 t/s | exact, HIT |
+| B2 | 196,960 | 98.0 s | 2,043 t/s | exact, HIT |
+| B3 | 196,961 | 201.1 s | 993 t/s | exact, HIT |
+| B4 | 196,960 | 149.9 s | 1,327 t/s | exact, HIT |
+| **total** | **787,842** | **238.4 s** | **3,305 t/s aggregate** | 4/4 |
+
+All four answered their own marker; nothing was preempted, refused, or truncated
+(`finish=stop`, `cache_n=0`, so no prefix cache padded the result). Aggregate
+prefill equals the single-stream rate measured in the same probe (3,397 t/s at
+43.5k), i.e. the four streams share one prefill pipeline at the slab-limited
+ceiling and the engine never ran short of KV. Prefix cache intact: the same
+43.5k prompt repeated answered in 1.0 s with `cache_n=43008`.
+
+The per-stream `decode` column is NOT a decode rate here — 8 generated tokens
+make it first-token dominated (the bench law: generate ≥ 128 tokens to read a
+rate). This probe answers one question only: does the endpoint owe four
+full-height conversations at once, and answer each one exactly. It does.
 
 ## Gates (measured at cutover)