fix(vegas): stop the heaviest plugin doubling across the cycle seam

Smooth Weighted Round-Robin spaces repeats well within a pass, but it
schedules the heaviest item first and usually last as well. The strip
loops, so those two are neighbours: the marquee showed the same plugin
twice running at exactly the one join a within-cycle check cannot see.
Observed on a live rig at 28 slots -- gaps of 6, 7, 7, 7 and then 1.

Rotating the list does not fix it. Rotation preserves the cyclic order
exactly, so it moves where the seam is drawn rather than the adjacency
itself; the trailing entry has to be swapped with one from the middle.

The first version swapped with the first slot that merely fitted, which
undid the spacing this exists to protect -- it moved a repeat from a gap
of 7 into a gap of 2, more clumped than the seam had ever been. It now
picks the candidate furthest from any other appearance, so the repeat
lands in the widest gap.

Left alone when no candidate exists. A plugin holding most of the slots
has to neighbour itself, and scheduling it is better than refusing to.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Udr6MfaFLUPhX5Fgo67Jf5

docs/ADVANCED_FEATURES.md

@@ -110,7 +110,13 @@ birds    > hockey > baseball
 ```
 
 18 slots for 12 plugins. Baseball appears 5 times, hockey 3, everything else
-once, and no plugin ever appears twice in a row.
+once, and no plugin ever appears twice in a row — **including across the seam**
+where the cycle loops back on itself. Smooth Weighted Round-Robin schedules the
+heaviest item first and usually last as well, so the strip would otherwise show
+it twice running at exactly the one join a within-cycle check cannot see. The
+trailing repeat is moved into the widest remaining gap. Where a double is
+unavoidable — a plugin holding most of the slots has to neighbour itself — the
+schedule is left as it is.
 
 #### Where the weight comes from