feat: Cincinnati post emits user variables as numbered #variables

When programs have user-defined variables, the Cincinnati post now:
- Assigns numbered machine variables (#200, #201, etc.) to non-inline variables
- Emits declarations like #200=48.0 (SHEET WIDTH) in the variable declaration subprogram
- Emits X#200 instead of X48.0 in coordinates that have VariableRefs
- Handles global variables (shared number across drawings) vs local (per-drawing number)
- Inline variables emit the literal value as before

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

OpenNest.Posts.Cincinnati/CincinnatiPostConfig.cs

@@ -253,6 +253,11 @@ namespace OpenNest.Posts.Cincinnati
             new() { MaxRadius = 4.500, FeedratePercent = 0.80, VariableNumber = 125 }
         };
 
+        [Category("A. Variables")]
+        [DisplayName("User Variable Start")]
+        [Description("Starting variable number for user-defined variables (#200, #201, etc.).")]
+        public int UserVariableStart { get; set; } = 200;
+
         [Category("A. Variables")]
         [DisplayName("Sheet Width Variable")]
         [Description("Variable number for sheet width.")]