fix(gpt6astra): rank lower Priority first and drop etch marks from geometry

Priority was sorted descending, the reverse of the host (StockLadder and
NestJobCandidateComparer treat a lower number as more important), so a
priority-9 part beat a priority-0 part for scarce stock. The existing test
encoded the inverted rule and now asserts the host's direction.

Part geometry filtered only rapids, so scribe/etch moves counted as
material - the bug OpenNest fixed in 1b5e1b1. Use SpecialLayers.IsMaterial.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

OpenNest.Engine.Gpt6Astra/README.md

@@ -20,8 +20,8 @@ concave interlocking, and insertion into straight-edged and curved holes.
    envelope area is within 8% of the minimum; retain additional orientations when needed to fit
    a candidate stock. Fixed and bounded rotation policies remain enforced. Bounded sweeps use
    up to 721 integer step indices, including permitted half-turn equivalents.
-3. Process high-priority parts first, then parts fitting fewer available stock types, then larger
-   envelopes. Larger frames precede inserts. Search every retained orientation for each instance.
+3. Process high-priority parts first (a lower `Priority` number ranks higher, as in the host),
+   then parts fitting fewer available stock types, then larger envelopes. Larger frames precede inserts. Search every retained orientation for each instance.
 4. Build cached Minkowski/no-fit regions. Convex pairs use Core's linear convex NFP primitive;
    concave pairs use Clipper's integer Minkowski sum. Arc-heavy concave contact outlines use
    a coarser mesh with both approximation bounds added to clearance; fine material geometry