FillPattern gathered per-angle results in a ConcurrentBag and kept the first best it
enumerated, so an exact tie between angles went to whichever worker finished first:
identical inputs could return different, equally scored layouts. Results now sit in
slots indexed by angle, V before H within an angle (the order the single-angle bag
already produced and the existing tie tests pin), and ties keep the earliest slot.
Single-angle behaviour is unchanged.
Regression: a never-prefer comparer over one 0-degree and fifteen 180-degree angles
must keep the 0-degree layout in 50 of 50 calls; it failed on the old code.
aj