File material extensions under their own name

An extension was written to the same path as the material it extends,
one pack over. A datapack file is identified by its path, so that never
added an extension to anything: it replaced the material with a stub,
whichever of the two packs loaded last. The stub then named itself as
its own parent, which MIAPI reads as a cycle and drops, and the material
was gone entirely for anyone with both mods installed:

  Unresolved extension cmmodular:metal/arcane_metal for target
  cmmodular:metal/arcane_metal (missing or cyclic dependency)

That is arcane metal and mithril, the two materials Create: Wizardry
adds forms of, and it explains why they worked before the extensions
existed. Ice and Fire's amethyst was never affected - it extends a
miapi: material, so its own cmmodular: path collided with nothing.

The generated name carries the pack that owns the extension, since the
pack is why the extension exists and there could be another from a
different mod later. Nothing else about the files changes.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tools/materials.py

@@ -1426,7 +1426,13 @@ def _extend_with_extra_forms(pack="create_wizardry"):
         path = f"{base['group']}/{name}"
         EXTENSIONS.append({
             "pack": pack,
-            "path": path,
+            # The extension is filed under its own name, not the material's. A
+            # datapack file is identified by its path, so writing it to the
+            # material's path in a second pack does not add an extension - it
+            # replaces the material with one, whichever pack loads last. The
+            # extension then names itself as its own parent, MIAPI reads that as
+            # a cycle, and the material is gone for anyone with both mods.
+            "path": f"{path}_from_{pack}",
             "data": {
                 "parent": f"cmmodular:{path}",
                 "data": {