frank-medium: double the host-RAM prompt cache to 16 GiB

--cache-ram caps the total bytes of saved prompt states in HOST RAM, and
alloc() refuses any single state larger than the whole cap. That refusal was
firing in production: 137 times in the two days from 2026-09-16, at 11.0-11.4
GiB per state, so every conversation past ~170k tokens was uncacheable and
re-prefilled from scratch on its next turn.

The state law at this shape is 68 KiB/token (16 full-attention layers x 4 kv
heads x 256 dim x 2 B x 2 for K/V, plus the MTP draft context's 4096 B), so
8192 MiB held ~120k tokens and 16384 holds ~240k: a FULL 196608-token slot
state (~12.8 GiB) now fits with ~3 GiB spare.

Host RAM only, allocated on demand and never pinned, so the VRAM fit is
untouched. New seam: the cap must clear one full slot's state and stay under
half the host's RAM (it fails on the old 8192 and passes on 16384). Also
corrects the adjacent claim that 0 is the build's default for
--ctx-checkpoints -- this binary's --help says 32.

scripts/frank-medium.env

@@ -245,11 +245,39 @@ THREAD_BATCH=8
 # f16 K/V: an agentic checkpoint is the wrong place to spend KV precision.
 CACHE_TYPE_K=f16
 CACHE_TYPE_V=f16
-# Context checkpoints: 0 is the build's default -- no per-slot KV snapshots, so
-# a re-sent or rewound conversation re-prefills instead of rewinding.  Stated
+# Context checkpoints: 0 -- no per-slot KV snapshots at all.  The build's
+# default is 32 (this binary's --help), so the 0 below is a deliberate
+# deviation, not an inherited default: a checkpoint is a VRAM->host-RAM copy
+# of part of a slot's KV, taken so a partially-matching prompt can be restored
+# instead of re-prefilled.  This endpoint trades that away -- its reuse comes
+# from the slot prompt and from the RAM-side prompt cache below.  Stated
 # explicitly so the tunable stays visible rather than merely absent.
 CTX_CHECKPOINTS=0
-CACHE_RAM=8192
+# PROMPT CACHE, HOST RAM -- this is the one cache tunable that is NOT VRAM.
+# --cache-ram caps the total bytes of saved prompt states
+# (server_prompt_cache, tools/server/server-task.cpp): update() evicts
+# oldest-first past the cap, and alloc() REFUSES outright any single state
+# bigger than the whole cap.  A state is the serialized KV of a whole sequence
+# plus its DeltaNet state plus the MTP draft context's own KV, so at this shape
+# it costs 68 KiB per token --
+#   16 full-attention layers x 4 kv heads x 256 dim x 2 B x 2 (K,V) = 64 KiB
+#   plus 4096 B for the draft context (one nextn layer)
+# -- which makes the cap a TOKEN budget in disguise: 8192 MiB held ~120k
+# tokens, 16384 MiB holds ~240k.
+#
+# DOUBLED 8192 -> 16384 on 2026-09-18 because the old cap was BINDING, not
+# theoretical: alloc() refused 137 states in the two days from 2026-09-16 with
+#   - prompt state size 11388.990 MiB exceeds cache size limit 8192.000 MiB, skipping
+# i.e. 11.0-11.4 GiB states -- conversations of ~170k tokens at the law above --
+# so every deep conversation was silently uncacheable and re-prefilled from
+# scratch on its next turn.  At 16384 a FULL 196608-token slot (~12.8 GiB) fits
+# with ~3 GiB of the budget spare for a second entry.
+#
+# The cost is host RAM, allocated on demand and never pinned (plain heap
+# vectors), so no VRAM number above moves.  dankserver has 62 GiB beside the
+# media-stack co-tenants, so the cache is bounded rather than unlimited (-1):
+# the seam test refuses a budget over half the host's RAM.
+CACHE_RAM=16384
 
 # Sampling defaults for requests that send none -- the values the vLLM
 # predecessor served on this endpoint (`--override-generation-config`), so a