Never place a chevron on the flat bottom threshold

pickChevrons() spaced chevrons evenly starting from whichever ring
block the flood-fill happened to discover first - an arbitrary point
depending on where the ring was punched, with no relationship to the
ring's actual top/bottom. That let spacing land a chevron directly on
the bottom row you walk through, which a real Stargate never has.

For a vertical ring, chevron spacing is now anchored at the true
topmost block and the bottom row is excluded from the candidate list
entirely before spacing runs. Horizontal rings are unaffected - they
have no "bottom" to avoid, so they still use the full ring.

README.md

@@ -36,10 +36,12 @@ doesn't try to flood-fill out through the open air in front of and behind the ga
 
 Chevrons aren't a separate material you place. When the gate is linked, the plugin
 flood-fills the ring, walks it in geometric order, and picks `gate.chevron-count`
-(default 7) blocks evenly spaced around the ring's actual perimeter. Those blocks
-swap to `gate.chevron-lit-material` (default glowstone) while dialing/open, and
-revert to whatever they looked like at rest when the gate closes - so a chevron can
-rest as plain obsidian and blend invisibly into the frame until it lights up.
+(default 7) blocks evenly spaced around the ring's actual perimeter - anchored at the
+topmost point for a vertical ring, and never the flat bottom row you walk through,
+matching a real Stargate. Those blocks swap to `gate.chevron-lit-material` (default
+glowstone) while dialing/open, and revert to whatever they looked like at rest when
+the gate closes - so a chevron can rest as plain obsidian and blend invisibly into
+the frame until it lights up.
 
 1. Build the ring.
 2. Optionally place a button directly below where you'll put the sign - this becomes