feat: add ReportGeneratorService and wire up in SimulateViewModel (#36)

- Generates simulation_report.md with config table, result, notes, full timeline
- Wired into SimulateViewModel after successful simulation
- Completes the Simulate Update feature (#31)

Author: JusterZhu

src/Services/ReportGeneratorService.cs

@@ -0,0 +1,64 @@
+using System;
+using System.IO;
+using System.Text;
+using System.Threading.Tasks;
+using GeneralUpdate.Tools.Models;
+
+namespace GeneralUpdate.Tools.Services;
+
+/// <summary>
+/// Generates simulation_report.md after a simulation run.
+/// </summary>
+public class ReportGeneratorService
+{
+    public async Task<string> GenerateAsync(
+        SimulateConfigModel config,
+        SimulationResult result,
+        string outputDir)
+    {
+        var sb = new StringBuilder();
+
+        sb.AppendLine("# Update Simulation Report");
+        sb.AppendLine();
+        sb.AppendLine("## Configuration");
+        sb.AppendLine();
+        sb.AppendLine("| Field | Value |");
+        sb.AppendLine("|-------|-------|");
+        sb.AppendLine($"| Patch | {EscapeMd(config.PatchFilePath)} |");
+        sb.AppendLine($"| App Directory | {EscapeMd(config.AppDirectory)} |");
+        sb.AppendLine($"| Platform | {config.Platform} |");
+        sb.AppendLine($"| AppType | {config.AppType} |");
+        sb.AppendLine($"| Version | {config.CurrentVersion} → {config.TargetVersion} |");
+        sb.AppendLine($"| Server Port | {config.ServerPort} |");
+        sb.AppendLine($"| Simulation Time | {DateTime.Now:yyyy-MM-dd HH:mm:ss} |");
+        sb.AppendLine();
+
+        sb.AppendLine("## Result");
+        sb.AppendLine();
+        sb.AppendLine($"**{(result.Success ? "✅ PASS" : "❌ FAIL")}** — {result.Elapsed.TotalSeconds:F1}s");
+        if (!string.IsNullOrEmpty(result.ErrorMessage))
+            sb.AppendLine($"\nError: `{result.ErrorMessage}`");
+        sb.AppendLine();
+
+        if (result.Notes.Count > 0)
+        {
+            sb.AppendLine("## Notes");
+            sb.AppendLine();
+            foreach (var note in result.Notes)
+                sb.AppendLine($"- {note}");
+            sb.AppendLine();
+        }
+
+        sb.AppendLine("## Timeline");
+        sb.AppendLine();
+        sb.AppendLine("```");
+        sb.Append(result.FullLog);
+        sb.AppendLine("```");
+
+        var reportPath = Path.Combine(outputDir, "simulation_report.md");
+        await File.WriteAllTextAsync(reportPath, sb.ToString(), Encoding.UTF8);
+        return reportPath;
+    }
+
+    private static string EscapeMd(string s) => s.Replace(@"\", @"\\").Replace("|", "\\|");
+}

src/ViewModels/SimulateViewModel.cs

@@ -13,6 +13,7 @@ public partial class SimulateViewModel : ViewModelBase
 {
     private readonly LocalizationService _loc = LocalizationService.Instance;
     private readonly SimulationService _sim = new();
+    private readonly ReportGeneratorService _report = new();
 
     public SimulateConfigModel Config { get; } = new();
 
@@ -85,8 +86,8 @@ async Task StartSimulation()
                     L($"  Note: {note}");
 
                 // Generate report
-                var reportPath = Path.Combine(Config.OutputDirectory, "simulation_report.md");
-                // report generation will be in next PR
+                var reportPath = await _report.GenerateAsync(Config, result, Config.OutputDirectory);
+                L($"Report: {reportPath}");
             }
             else
             {