frank-default-small: 491520 context pool (163840 per slot x 3)

--ctx-size is the total pool divided among --parallel slots, so with
PARALLEL=3 the 160k target must be a 491520 pool (163840/slot) to
actually deliver 160k context per slot. Env, serve default, bats updated.

scripts/serve-frank-small.sh

@@ -49,7 +49,7 @@ fi
 : "${ALIAS:=frank-default-small}"
 : "${BIND_ADDR:=0.0.0.0}"
 : "${LISTEN_PORT:=7770}"
-: "${CTX_SIZE:=160000}"
+: "${CTX_SIZE:=491520}"
 : "${PARALLEL:=3}"
 : "${SPEC_TYPE:=none}"
 : "${CACHE_TYPE_K:=q8_0}"