refactor: extract training data collection into OpenNest.Training

Move brute-force data collection, TrainingDatabase, and GPU init from
OpenNest.Console into a dedicated OpenNest.Training project. Replaces
raw Microsoft.Data.Sqlite with EF Core. Console is now a pure nesting
CLI with template support and cleaned-up usage output.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Training/Data/TrainingDbContext.cs

@@ -0,0 +1,38 @@
+using Microsoft.EntityFrameworkCore;
+
+namespace OpenNest.Training.Data
+{
+    public class TrainingDbContext : DbContext
+    {
+        public DbSet<TrainingPart> Parts { get; set; }
+        public DbSet<TrainingRun> Runs { get; set; }
+
+        private readonly string _dbPath;
+
+        public TrainingDbContext(string dbPath)
+        {
+            _dbPath = dbPath;
+        }
+
+        protected override void OnConfiguring(DbContextOptionsBuilder options)
+        {
+            options.UseSqlite($"Data Source={_dbPath}");
+        }
+
+        protected override void OnModelCreating(ModelBuilder modelBuilder)
+        {
+            modelBuilder.Entity<TrainingPart>(e =>
+            {
+                e.HasIndex(p => p.FileName).HasDatabaseName("idx_parts_filename");
+            });
+
+            modelBuilder.Entity<TrainingRun>(e =>
+            {
+                e.HasIndex(r => r.PartId).HasDatabaseName("idx_runs_partid");
+                e.HasOne(r => r.Part)
+                    .WithMany(p => p.Runs)
+                    .HasForeignKey(r => r.PartId);
+            });
+        }
+    }
+}