frank-primary: persist rad-tune kernel cache on a host mount

The engine's kernel selector reads $RADIANCE_HOME/tune/<machine>.tune at
boot; $RADIANCE_HOME defaults to /opt/radiance, which lives in the image,
so a cache written by a --rm scratch container dies with it. Pre-create
$FRANK_PRIMARY_STATE_DIR/tune and bind-mount it at
/opt/radiance/share/radiance/tune so the rad-tune cache (generated in a
stopped-service window) survives and is read at every production boot.

scripts/frank-primary.env

@@ -59,6 +59,11 @@ FRANK_PRIMARY_IMAGE=stilldeadcode/radiance@sha256:4c3f484564bb2872784e46b0a59cbd
 FRANK_PRIMARY_MODEL_DIR=/home/dank/models/rad
 FRANK_PRIMARY_MODEL_FILE=qwen4exp-fp8-native.rad
 FRANK_PRIMARY_STATE_DIR=/home/dank/radiance-state
+# The state dir's tune/ subdirectory holds rad-tune's kernel-selection
+# cache (the engine's selector reads $RADIANCE_HOME/tune/<machine>.tune
+# at boot; $RADIANCE_HOME defaults to /opt/radiance, in the image, so the
+# cache must ride a host mount — serve script mounts it, see the runbook's
+# "Tuning levers").
 # Container identity is the SERVICE name (Frank's call, 2026-10-05): one
 # name across systemctl and podman ps. The runtime changes; the name does
 # not. The legacy list retires every predecessor identity that could hold