log: frank-medium prompt cache doubled to 16 GiB

serverAppConfigs-docs/log.md

@@ -941,3 +941,31 @@ timestamp: 2026-09-14
   `ps`/`ss` and prove it abandoned (frozen CPU ticks, no conns) before
   reclaiming. Scratch servers on loopback are invisible to the endpoint
   monitor, so they strand the fleet instead of failing loudly.
+
+## 2026-09-18 (afternoon) — frank-medium: the host-RAM prompt cache doubles to 16 GiB
+
+- **The old cap was binding, not theoretical.** `--cache-ram` caps the total
+  bytes of saved prompt states in HOST RAM (`server_prompt_cache`), and
+  `alloc()` refuses any single state larger than the whole cap with one `WRN`.
+  The journal held **137** such refusals in the two days from 2026-09-16 —
+  `prompt state size 11388.990 MiB exceeds cache size limit 8192.000 MiB,
+  skipping` — i.e. every conversation past ~170k tokens was uncacheable, and
+  its next turn re-prefilled the whole thing.
+- **The cap is a token budget in disguise.** A state is the serialized KV of a
+  whole sequence plus its constant-size DeltaNet state plus the MTP draft
+  context's own KV: 16 full-attention layers x 4 kv heads x 256 dim x 2 B x 2
+  (K,V) = 64 KiB per token, plus 4096 B for the draft context. So 8192 MiB held
+  ~120k tokens and 16384 MiB holds ~240k — and a FULL 196608-token slot state
+  (~12.8 GiB), which the old cap refused outright, now fits with ~3 GiB spare.
+- **Deployed change** (`1acca43`): `CACHE_RAM` 8192 → 16384 in
+  `scripts/frank-medium.env`, with the law and the journal evidence in the
+  file. Host RAM, allocated on demand and never pinned, so no VRAM number
+  moves; bounded rather than `-1` because dankserver shares its 62 GiB with the
+  media-stack co-tenants.
+- **New seam** (`tests/frank_medium_unit.bats`): the cap must clear one full
+  slot's state and may not claim more than half the host's RAM. It fails on the
+  old value (8192 < 13056 MiB) and passes on the new one — checked both ways.
+- **Adjacent correction:** the same block claimed 0 was the build's default for
+  `--ctx-checkpoints`; this binary's `--help` says **32**, so the 0 is a
+  deliberate deviation. Fixed in the env comment and in the runbook's flag
+  table.