test(editor): make control assertions handle-state independent and order-proving

Quality review of 50884da found the unshown RichTextBox oracle compared
cached CRLF text against the highlighted editor's native LF-normalized
text, which would fail on first Windows execution. Compare generated
text with normalized line endings on both sides instead, capture the
status label and preview state inside the ProgramChanged handler to
prove fallback-before-notify ordering, document the HighlightSpan
UTF-16 and rule-index contract, and pin the null-text argument check.

OpenNest.Core/CNC/ProgramHighlighting.cs

@@ -4,6 +4,15 @@ using System.Text.RegularExpressions;
 
 namespace OpenNest.CNC;
 
+/// <summary>
+/// A character range in generated G-code assigned by one highlighting rule.
+/// </summary>
+/// <param name="Index">Zero-based UTF-16 index of the first colored character.</param>
+/// <param name="Length">Number of UTF-16 characters to color.</param>
+/// <param name="RuleIndex">
+/// Zero-based rule index: 0 comments, 1 motion modes (G90/G91), 2 rapid moves (G00),
+/// 3 linear moves (G01), 4 arcs (G02/G03).
+/// </param>
 public readonly record struct HighlightSpan(int Index, int Length, int RuleIndex);
 
 /// <summary>Computes cosmetic G-code highlight spans without changing the generated text.</summary>