Builder: seed third-party without gitfile; skip init when pre-seeded.

Isolated Docker builds copy native deps from host without broken submodule
.git pointers; CI init script skips network fetch when trees validate.

Co-authored-by: Cursor <cursoragent@cursor.com>

init-third-party-submodules.sh

@@ -91,6 +91,11 @@ for path in "${SUB_PATHS[@]}"; do
     echo "error: $path not in .gitmodules" >&2
     exit 1
   fi
+  if validate_path "$path"; then
+    short="$(git -C "$path" rev-parse --short HEAD 2>/dev/null || echo seeded)"
+    echo "    $path @ $short (pre-seeded)"
+    continue
+  fi
   ensure_gitlink "$path"
   git submodule sync "$path"
   git submodule update --init --recursive "$path"