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.

README.md

@@ -16,12 +16,15 @@ Build everything with `./gradlew build`. Jars land in each module's `build/libs/
 
 ## Building a gate
 
-1. Build a ring out of any block in `gate.frame-materials` (default: obsidian, gold
-   block, birch/oak planks — matches an obsidian+gold arch design). Leave the middle
-   hollow.
-2. Embed a few blocks of `gate.chevron-unlit-material` (default black stained glass)
-   into the ring — these light up to `gate.chevron-lit-material` (default glowstone)
-   as the gate dials.
+1. Build a fully-enclosed ring out of any block(s) in `gate.frame-materials` (default:
+   obsidian, gold block, gilded blackstone, birch/oak planks). Leave the middle hollow.
+   No gaps - the ring must be a sealed loop.
+2. Nothing else to build for chevrons. On creation, the plugin automatically picks
+   `gate.chevron-count` (default 6) frame blocks, evenly spaced around the ring, and
+   remembers what they looked like at rest. While dialing/open those blocks swap to
+   `gate.chevron-lit-material` (default gilded blackstone); closing the gate restores
+   them to their original block, so a chevron can rest as plain obsidian and blend
+   invisibly into the frame until it lights up.
 3. Place a sign on the outside of the frame with:
    - Line 1: `[Stargate]`
    - Line 2: network name (blank = default network)