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.
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.