refactor: rename Astra engine to Gpt6Astra

OpenAI reuses model codenames across generations (GPT-6 reused
GPT-5.6's Sol and Luna), so a bare codename like "Astra" can't
identify which model built the engine. Prefixing the model version
keeps engine names unambiguous as more runs are added. The CLR type
is now Gpt6AstraNestingEngine, so benchmark reports show the new name.

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

OpenNest.Engine.Gpt6Astra/benchmarks/results/baseline-generated.csv

@@ -0,0 +1,24 @@
+case,valid,placed,requested,sheets,area,milliseconds
+generated-library-0-RoundedRectangle,True,24,24,2,1886,24
+generated-library-1-T,True,24,24,3,5917,0
+generated-library-2-Trapezoid,True,24,24,2,2150,0
+generated-library-3-Ngon,True,24,24,2,2024,0
+generated-library-4-Ngon,True,24,24,2,2160,0
+generated-library-5-Ring,True,24,24,1,3901,3
+generated-library-6-PipeFlange,True,12,12,2,1932,19
+generated-ring-inserts,True,16,16,2,2016,2
+generated-curved-C,True,16,16,3,6051,0
+generated-narrow-U,True,24,24,3,5925,0
+generated-stars,True,20,20,1,3901,0
+generated-seed-00,True,19,19,2,1886,1
+generated-seed-01,True,22,22,2,2016,1
+generated-seed-02,True,21,21,2,2150,1
+generated-seed-03,True,20,20,1,1012,1
+generated-seed-04,True,24,24,1,1080,1
+generated-seed-05,True,22,22,2,2050,1
+generated-seed-06,True,27,27,2,1932,1
+generated-seed-07,True,19,19,1,1032,1
+generated-seed-08,True,17,17,1,1100,1
+generated-seed-09,True,22,22,2,2070,1
+generated-seed-10,True,20,20,2,1968,1
+generated-seed-11,True,19,19,2,2100,1