feat(apps): AppInstaller writes a diagnostic log (winget resolve + bootstrap + per-app)

Writes C:\ProgramData\SilverMetal\appinstall.log (best-effort) so a post-install mount
shows exactly where app installs fail: winget probe results, bootstrap-winget output,
and per-app winget exit codes. Makes the no-apps-installed failure diagnosable instead
of inferred.

windows/welcome/src/SilverOS.Welcome.Core/Apps/AppInstaller.cs

@@ -4,17 +4,38 @@ namespace SilverOS.Welcome.Core.Apps;
 
 public sealed class AppInstaller(IProcessRunner runner, string appsDir) : IAppInstaller
 {
+    // Best-effort diagnostic log (winget resolution, bootstrap output, per-app results).
+    // Lives under ProgramData so it survives + is readable post-install. Never throws.
+    private static readonly string LogPath = Path.Combine(
+        Environment.GetFolderPath(Environment.SpecialFolder.CommonApplicationData), "SilverMetal", "appinstall.log");
+
+    private static void Log(string msg)
+    {
+        try
+        {
+            Directory.CreateDirectory(Path.GetDirectoryName(LogPath)!);
+            File.AppendAllText(LogPath, $"{DateTime.Now:HH:mm:ss.fff} {msg}{Environment.NewLine}");
+        }
+        catch { /* logging is best-effort */ }
+    }
+
+    private static string Snip(string? s) =>
+        string.IsNullOrWhiteSpace(s) ? "" : s.Trim().Replace("\r", " ").Replace("\n", " ") is var t && t.Length > 300 ? t[..300] : t;
+
     public async Task<IReadOnlyList<AppInstallResult>> InstallAsync(
         IReadOnlyList<AppCatalogEntry> apps, IProgress<ApplyProgress> progress, CancellationToken ct = default)
     {
         var results = new List<AppInstallResult>();
         if (apps.Count == 0) return results;
 
+        Log($"InstallAsync: {apps.Count} app(s) requested: {string.Join(", ", apps.Select(a => a.Id))}");
+
         // App installs are non-critical: a missing/broken winget (e.g. offline IoT LTSC) must
         // NEVER fail onboarding. Resolve winget defensively; if it can't be found, skip installs.
         var winget = await ResolveWingetAsync(progress, ct);
         if (winget is null)
         {
+            Log($"winget UNAVAILABLE -> skipping all {apps.Count} app(s)");
             progress.Report(new($"App installer unavailable - skipping {apps.Count} app(s)", 80));
             foreach (var app in apps) results.Add(new AppInstallResult(app.Id, false));
             return results;
@@ -33,6 +54,7 @@ public sealed class AppInstaller(IProcessRunner runner, string appsDir) : IAppIn
                     $"install --id {id} --silent --accept-package-agreements --accept-source-agreements --disable-interactivity",
                     ct);
                 ok = r.ExitCode == 0;
+                Log($"install {id}: exit={r.ExitCode} ok={ok} err={Snip(r.StdErr)}");
                 if (ok && !string.IsNullOrWhiteSpace(app.Configure))
                 {
                     var script = Path.Combine(appsDir, "configure", app.Configure);
@@ -43,6 +65,7 @@ public sealed class AppInstaller(IProcessRunner runner, string appsDir) : IAppIn
             }
             results.Add(new AppInstallResult(app.Id, ok));
         }
+        Log($"InstallAsync done: {results.Count(r => r.Installed)}/{results.Count} installed");
         return results;
     }
 
@@ -52,22 +75,30 @@ public sealed class AppInstaller(IProcessRunner runner, string appsDir) : IAppIn
     private async Task<string?> ResolveWingetAsync(IProgress<ApplyProgress> progress, CancellationToken ct)
     {
         // 1) Already launchable by name (on PATH for this process)?
-        if ((await TryRunAsync("winget", "--version", ct)).ExitCode == 0) return "winget";
+        var p1 = await TryRunAsync("winget", "--version", ct);
+        Log($"winget probe (PATH): exit={p1.ExitCode} out={Snip(p1.StdOut)}");
+        if (p1.ExitCode == 0) return "winget";
 
         // 2) Provision App Installer via the bundled bootstrap (or registered package), then re-probe.
         progress.Report(new("Preparing app installer", 68));
         var bootstrap = Path.Combine(appsDir, "bootstrap-winget.ps1");
-        await TryRunAsync("powershell.exe",
+        var b = await TryRunAsync("powershell.exe",
             $"-NoProfile -ExecutionPolicy Bypass -Command \"if (Test-Path '{bootstrap}') {{ & '{bootstrap}' }} else {{ " +
             "Add-AppxPackage -RegisterByFamilyName -MainPackage Microsoft.DesktopAppInstaller_8wekyb3d8bbwe -EA SilentlyContinue }}\"",
             ct);
-        if ((await TryRunAsync("winget", "--version", ct)).ExitCode == 0) return "winget";
+        Log($"bootstrap-winget: exit={b.ExitCode} out={Snip(b.StdOut)} err={Snip(b.StdErr)}");
+        var p2 = await TryRunAsync("winget", "--version", ct);
+        Log($"winget probe (post-bootstrap): exit={p2.ExitCode} out={Snip(p2.StdOut)}");
+        if (p2.ExitCode == 0) return "winget";
 
         // 3) Fall back to the WindowsApps execution-alias path (bare-name launch can fail under
         //    UseShellExecute=false even when winget is installed).
         var local = Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData);
         var aliased = Path.Combine(local, "Microsoft", "WindowsApps", "winget.exe");
-        if (File.Exists(aliased) && (await TryRunAsync(aliased, "--version", ct)).ExitCode == 0) return aliased;
+        var aliasExists = File.Exists(aliased);
+        var p3Exit = aliasExists ? (await TryRunAsync(aliased, "--version", ct)).ExitCode : -1;
+        Log($"winget alias path '{aliased}': exists={aliasExists} probe={p3Exit}");
+        if (aliasExists && p3Exit == 0) return aliased;
 
         return null;
     }