perf(welcome): cut first-boot cold-start + add loading affordance

The Welcome wizard showed nothing until WebView2 cold-started and Blazor
booted, so the whole startup cost presented as a blank window long enough
that operators thought first boot had failed.

- Native MAUI splash overlay (renders in the first frame, no WebView2/JIT
  dependency) + a visually identical in-page splash inside #app, so the
  native -> webview -> Blazor handoff reads as one continuous loading
  screen. Fades out on first successful WV2 NavigationCompleted.
- PublishReadyToRun=true (publish-only) to remove first-run JIT on the
  one-shot cold-disk path. R2R header verified present after publish.
- Fixed-version WebView2 runtime baked offline next to the exe (build.ps1
  stages it, app points WEBVIEW2_BROWSER_EXECUTABLE_FOLDER at it). Removes
  the Evergreen registry probe and the LTSC "no WebView2 at all" risk flagged
  in welcome-app-spec.md; air-gap friendly. Absent => falls back to Evergreen.
- De-flash launch: drop the `cmd /c` wrapper and add -WindowStyle Hidden in
  autounattend FirstLogonCommands (kills the console flash + one process).

Verified: Release build clean, win-x64 self-contained publish succeeds with
R2R confirmed, 38/38 tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

windows/installer/build.ps1

@@ -183,6 +183,22 @@ function Copy-WelcomePayload {
     } else {
         Write-Warning "   No apps dir found at $appsDir -- image will ship with no app catalog."
     }
+    # Stage the fixed-version WebView2 runtime, if vendored, next to the app.
+    # Cold-start + air-gap: a fixed-version runtime is just files (no installer
+    # step at first boot) and removes the dependency on whether IoT Enterprise LTSC
+    # ships WebView2 at all. Operator populates windows\welcome\runtime\webview2\
+    # with an EXTRACTED "Microsoft Edge WebView2 Fixed Version" distribution (the
+    # folder that contains msedgewebview2.exe) -- handled like the drivers dir:
+    # absent is allowed (VM/dev test), in which case the app falls back to Evergreen.
+    $wv2Src = Join-Path $WindowsDir 'welcome\runtime\webview2'
+    if (Test-Path (Join-Path $wv2Src 'msedgewebview2.exe')) {
+        $wv2Dest = Join-Path $dest 'webview2'
+        $null = New-Item -ItemType Directory -Force $wv2Dest
+        Copy-Item (Join-Path $wv2Src '*') $wv2Dest -Recurse -Force
+        Write-Host "   Staged fixed-version WebView2 runtime to $wv2Dest"
+    } else {
+        Write-Warning "   No fixed-version WebView2 runtime at $wv2Src (expected msedgewebview2.exe) -- image will rely on the Evergreen runtime being present at first boot. See windows\welcome\runtime\webview2\README.md."
+    }
     # --- Guard: verify the payload actually landed in the mounted image -------
     $stagedExe = Join-Path $dest 'SilverOS.Welcome.App.exe'
     if (-not (Test-Path $stagedExe)) {