Narrow default frame-materials to just obsidian

GOLD_BLOCK/GILDED_BLACKSTONE/BIRCH_PLANKS/OAK_PLANKS were also present
on a nearby backdrop wall in the user's build, so even with the
plane-locked diagonal fix (which only stops leaks through the ring's
thickness), the flood-fill could still sweep in decorative use of the
same materials sitting in-plane with the ring, putting chevrons on
unrelated blocks. Default is now just OBSIDIAN; anyone mixing
materials should keep the list to ring-only blocks per the updated
config.yml/README guidance.

stargate-paper/src/main/resources/config.yml

@@ -28,19 +28,16 @@ cross-server:
 gate:
   # Any of these materials count as the ring's frame (and count as a valid chevron
   # material too, since chevrons are just frame blocks the plugin temporarily swaps).
-  # Mix and match to match your build (the default matches an obsidian/gilded-blackstone
-  # ring). The ring can be any size or shape, as long as it's a single connected loop
-  # made only of these materials with a fully sealed hollow interior - no fixed template
-  # or size is required. "Connected" includes diagonal stepping (round rings need that
-  # at their corners), but only within the ring's own flat plane - a nearby structure
-  # built from the same materials won't get pulled in just because it touches the ring
-  # through its thickness.
+  # Add to this list to match your build - but keep it to materials that are ONLY
+  # used for the ring itself. Anything also used decoratively nearby (a backdrop
+  # wall, a matching accent block elsewhere) risks the flood-fill picking up that
+  # unrelated structure as part of the frame if it touches the ring anywhere in the
+  # same plane. The ring can be any size or shape, as long as it's a single connected
+  # loop with a fully sealed hollow interior - no fixed template or size is required.
+  # "Connected" includes diagonal stepping (round rings need that at their corners),
+  # but only within the ring's own flat plane.
   frame-materials:
     - OBSIDIAN
-    - GOLD_BLOCK
-    - GILDED_BLACKSTONE
-    - BIRCH_PLANKS
-    - OAK_PLANKS
   # Chevrons aren't a separate material you place. The plugin flood-fills the ring,
   # walks it in geometric order, and picks this many blocks, evenly spaced around the
   # ring's actual perimeter (not raw scan order) - then swaps just those to