perf: use EnumerateLines for line splitting in HtmlReportGenerator (#6089)

* perf: use EnumerateLines for line splitting in HtmlReportGenerator

Replace `message.Replace("\r\n", "\n").Split('\n')` with span-based
`MemoryExtensions.EnumerateLines()` on net8+, eliminating the Replace
string and Split array allocations per rendered test result. The
netstandard2.0 path retains the existing Replace/Split fallback.

Behavior is identical: both code paths match the same line prefixes.

Closes #6034

* perf: early-exit TryExtractExpectedActual once both values found

Address review on PR #6089: stop scanning the assertion message as soon as
both the Expected and Actual lines have been captured, instead of iterating
every remaining line (stack traces, diff blocks). Applied to both the #if NET
EnumerateLines path and the netstandard2.0 Split fallback.

Author: thomhurst

TUnit.Engine/Reporters/Html/HtmlReportGenerator.cs

@@ -672,6 +672,27 @@ private static void TryExtractExpectedActual(string message, out string? expecte
         expected = null;
         actual = null;
         if (string.IsNullOrEmpty(message)) return;
+#if NET
+        // EnumerateLines splits on \n and \r\n natively — no Replace/Split allocations.
+        foreach (var rawLine in message.AsSpan().EnumerateLines())
+        {
+            var line = rawLine.TrimStart();
+            if (expected is null && line.StartsWith("Expected:", StringComparison.OrdinalIgnoreCase))
+            {
+                expected = line.Slice("Expected:".Length).Trim().ToString();
+            }
+            else if (actual is null && (line.StartsWith("Actual:", StringComparison.OrdinalIgnoreCase) || line.StartsWith("But was:", StringComparison.OrdinalIgnoreCase)))
+            {
+                var prefixLen = line.StartsWith("Actual:", StringComparison.OrdinalIgnoreCase) ? "Actual:".Length : "But was:".Length;
+                actual = line.Slice(prefixLen).Trim().ToString();
+            }
+
+            if (expected is not null && actual is not null)
+            {
+                break;
+            }
+        }
+#else
         var lines = message.Replace("\r\n", "\n").Split('\n');
         foreach (var raw in lines)
         {
@@ -685,7 +706,13 @@ private static void TryExtractExpectedActual(string message, out string? expecte
                 var prefixLen = line.StartsWith("Actual:", StringComparison.OrdinalIgnoreCase) ? "Actual:".Length : "But was:".Length;
                 actual = line.Substring(prefixLen).Trim();
             }
+
+            if (expected is not null && actual is not null)
+            {
+                break;
+            }
         }
+#endif
         if (expected is { Length: 0 }) expected = null;
         if (actual is { Length: 0 }) actual = null;
     }
@@ -767,14 +794,24 @@ private static bool IsDuplicateKey(ReportKeyValue[] items, int index, string key
     {
         if (string.IsNullOrEmpty(stderr)) return stderr;
         if (stderr!.IndexOf("[TUnit]", StringComparison.Ordinal) < 0) return stderr;
-        var lines = stderr.Replace("\r\n", "\n").Split('\n');
         var sb = new StringBuilder(stderr.Length);
+#if NET
+        // EnumerateLines splits on \n and \r\n natively — no Replace/Split allocations.
+        foreach (var line in stderr.AsSpan().EnumerateLines())
+        {
+            if (line.TrimStart().StartsWith("[TUnit]", StringComparison.Ordinal)) continue;
+            if (sb.Length > 0) sb.Append('\n');
+            sb.Append(line);
+        }
+#else
+        var lines = stderr.Replace("\r\n", "\n").Split('\n');
         foreach (var line in lines)
         {
             if (line.TrimStart().StartsWith("[TUnit]", StringComparison.Ordinal)) continue;
             if (sb.Length > 0) sb.Append('\n');
             sb.Append(line);
         }
+#endif
         return sb.Length == 0 ? null : sb.ToString();
     }