frank-large: lower KV slots 4 -> 3

Concurrency cap trimmed to the real fleet size. Each slot is 111.0 MiB of
O(1) state, so this hands ~111 MiB back on a host sitting at 7.1 GiB left
for everything else.

Note the slots already oversubscribed the pool (4 x 262144 > 524288), so
admission was pool-bound regardless: at most two full-length conversations,
or four at 131k. This does not change how much context fits -- it caps
simultaneous requests at three, and a fourth now queues.

dankclaw/frank-large/quadlet/frank-large.container

@@ -100,7 +100,15 @@ Environment=HALOGEN_CTX=262144
 # Doubled 2026-09-12 from the 262144 this had run since 09-11: two full-length
 # conversations resident at once, or four at 131k, instead of one.
 Environment=HALOGEN_KV_POOL_POSITIONS=524288
-Environment=HALOGEN_KV_SLOTS=4
+# Concurrency cap. Each slot is 111.0 MiB of O(1) state; the KV is ONE
+# 524288-position pool carved into per-request regions, so slots cap how many
+# requests run at once while the pool binds how many positions exist. These
+# slots OVERSUBSCRIBE the pool (4 x 262144 > 524288), so admission was always
+# pool-bound -- at most two full-length conversations, or four at 131k.
+# LOWERED 4 -> 3 on 2026-09-12: frees ~111 MiB on a host with 7.1 GiB left,
+# and three users is the real fleet size. A fourth concurrent request now
+# queues behind the three rather than contending.
+Environment=HALOGEN_KV_SLOTS=3
 # Making the doubling stick is budget arithmetic, all of it. ONE budget covers
 # the KV pool, the prefill arena and the vision tower together:
 #