Add gilded blackstone to default frame materials, report scan failure reasons

Sign creation now tells the player exactly why the structure scan failed
(missing frame material, ring not sealed, interior too large, etc.)
instead of one generic error.

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

@@ -31,6 +31,7 @@ gate:
   frame-materials:
     - OBSIDIAN
     - GOLD_BLOCK
+    - GILDED_BLACKSTONE
     - BIRCH_PLANKS
     - OAK_PLANKS
   # Chevron blocks embedded in the frame. Build them as chevron-unlit-material; the