fix(compiler): add diagnostic explanations for SPY0026 and SPY0027

Add DiagnosticExplanation entries for the two new byte string diagnostic
codes (UnterminatedByteString, UnicodeEscapeInByteString) added in the
byte literal lexer support. Update active code count comment.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: antonsynd

src/Sharpy.Compiler/Diagnostics/DiagnosticCodes.cs

@@ -24,8 +24,8 @@ public static class DiagnosticCodes
 {
     /// <summary>
     /// Lexer diagnostic codes (SPY0001-SPY0099).
-    /// Active: SPY0001-SPY0025 (25 codes)
-    /// Reserved: SPY0026-SPY0099 (74 codes for future lexer diagnostics)
+    /// Active: SPY0001-SPY0027 (27 codes)
+    /// Reserved: SPY0028-SPY0099 (72 codes for future lexer diagnostics)
     /// </summary>
     public static class Lexer
     {

src/Sharpy.Compiler/Diagnostics/DiagnosticExplanations.cs

@@ -167,6 +167,16 @@ private static Dictionary<string, DiagnosticExplanation> BuildExplanations()
             "import `System.IO`",
             "Split into separate backtick segments:\n  import `System`.IO");
 
+        Add(dict, DiagnosticCodes.Lexer.UnterminatedByteString, "Unterminated byte string literal", "Lexer",
+            "A byte string literal was opened with b\" or b' but never closed. The lexer reached the end of the line or file without finding a matching closing quote.",
+            "data: bytes = b\"hello",
+            "Add the closing quote:\n  data: bytes = b\"hello\"");
+
+        Add(dict, DiagnosticCodes.Lexer.UnicodeEscapeInByteString, "Unicode escape in byte string", "Lexer",
+            "A byte string literal contains a \\u or \\U unicode escape sequence, which is not allowed. Byte strings can only contain values in the 0-255 range. Use \\x escapes for hex byte values instead.",
+            "data: bytes = b\"\\u0041\"",
+            "Use a hex escape instead:\n  data: bytes = b\"\\x41\"");
+
         // ── Parser errors (SPY0100-SPY0199) ─────────────────────────────
 
         Add(dict, DiagnosticCodes.Parser.UnexpectedToken, "Unexpected token", "Parser",