scripts: paths from the environment, not one disk

scripts/react_paths.py

@@ -63,3 +63,13 @@ def require(path: Path, what: str, env: str) -> Path:
             f"(REACT_RAW is the original HDF5 capture tree, which the release "
             f"does not publish — steps needing it are marked in the README.)")
     return Path(path)
+
+
+def out_root(name: str) -> Path:
+    """Where a build script writes its artefacts.
+
+    `REACT_OUT` or the working layout. Build scripts wrote to absolute paths
+    on one disk, which made them unpublishable: a reader could run them and
+    get a permission error rather than a page.
+    """
+    return Path(os.environ.get("REACT_OUT", "/media/yxma/Disk1/twm")) / name