refact(frank-dragoon): serve gates run on the shared gate library

The hand-written image/ck/tokenizer gates are require_image, require_bytes and
require_dir_files from scripts/lib/llm-gates.sh. The serve seam is unchanged
and still green (its podman stub now models `podman image exists`, the
primitive the gate uses). Behaviour preserved: every refusal names the
artifact, the endpoint and the fetch script.

scripts/lib/llm-gates.sh

@@ -69,12 +69,14 @@ require_min_mib() {
 
 # require_dir_files — DIR must be a directory and every named member a
 # regular file (the flat-tokenizer law: snapshot symlinks dangle in the
-# container mount even when the directory itself is real).
+# container mount even when the directory itself is real. A symlinked
+# DIRECTORY is refused too: a HF cache snapshot can link the directory
+# itself, and a link dangles inside the container mount.
 require_dir_files() {
     local dir="$1" label="$2" hint="$3"
     shift 3
     local f
-    if [[ ! -d "$dir" ]]; then
+    if [[ -L "$dir" || ! -d "$dir" ]]; then
         _gate_fail "$label: missing at $dir" "$hint"
     fi
     for f in "$@"; do