frank-primary: never write the stage index through the raw symlink

Run 3 rewrote the staged index THROUGH the relative symlink into the raw
checkpoint: the raw index.json was restamped with the stacked layout
(605 KB smaller), the raw verify then failed 143/144 and the overnight
run died before stacking. Two fixes:

- stack-qwen38-official-fp8.py: index/config writes are now temp+os.replace
  (an atomic swap replaces the symlink itself; the raw tree stays pristine).
- build-frank-primary-rad.sh: the stage's index.json is COPIED like
  config.json; only shards stay relative symlinks.

scripts/build-frank-primary-rad.sh

@@ -98,13 +98,18 @@ dropped = cfg.pop("quantization_config", None)
 json.dump(cfg, open(dst, "w"), indent=2)
 print(f"staging config.json written (quantization_config dropped: {dropped is not None})")
 PY
-    # Everything else is a RELATIVE symlink — relative so the targets resolve
-    # inside the convert container (the stage's parent is mounted at /models).
+    # The index is COPIED like config.json, never symlinked: the reshape step
+    # rewrites the stage's index, and a symlink would write through to the
+    # raw checkpoint (measured 2026-10-05: the raw index was restamped with
+    # the stacked layout and the raw verify failed 143/144). Shards stay
+    # relative symlinks — relative so the targets resolve inside the convert
+    # container (the stage's parent is mounted at /models).
     for f in "$SRC"/*; do
         name="$(basename "$f")"
-        [ "$name" = "config.json" ] && continue
+        case "$name" in config.json|model.safetensors.index.json) continue ;; esac
         ln -sfn "../Qwen/Qwen3.8-Flash-Next-FP8/$name" "$STAGE/$name"
     done
+    cp "$SRC/model.safetensors.index.json" "$STAGE/"
     echo "staging dir ready: $STAGE ($(ls "$STAGE" | wc -l) entries)"
 fi