Auto-select chevron slots instead of requiring a distinct unlit material

Chevrons no longer need to be built out of a special block - the plugin
picks gate.chevron-count frame blocks evenly spaced around the scanned
ring and remembers their resting material, so a chevron can rest as
plain obsidian (matching the rest of the frame) and only reveal itself
by swapping to gate.chevron-lit-material while dialing/open. Restoring
on close now puts back the original block instead of a fixed material,
so alternating decorative frames (e.g. obsidian/gilded blackstone) are
preserved exactly.

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

@@ -34,11 +34,13 @@ gate:
     - GILDED_BLACKSTONE
     - BIRCH_PLANKS
     - OAK_PLANKS
-  # Chevron blocks embedded in the frame. Build them as chevron-unlit-material; the
-  # plugin swaps them to chevron-lit-material as the gate dials/opens, and swaps them
-  # back when the gate closes/deactivates.
-  chevron-unlit-material: BLACK_STAINED_GLASS
-  chevron-lit-material: GLOWSTONE
+  # Chevrons are NOT a separate material you have to place - the plugin automatically
+  # picks this many frame blocks, evenly spaced around the ring, and swaps them to
+  # chevron-lit-material while dialing/open, then restores whatever they looked like
+  # at rest when the gate closes. So a chevron can rest as plain obsidian and only
+  # stand out once it lights up.
+  chevron-lit-material: GILDED_BLACKSTONE
+  chevron-count: 6
   # Material poured into the interior (the "event horizon") while the gate is open.
   iris-material: WATER
   max-frame-blocks: 300