Engineer image: upload the Engineering Layer from the web app

New Engineering Layer tab on the engineer image page. A site uploads
Engineering_Install_v<ver>.exe (keeps prereqs) or a .zip of the whole
extra_engineering payload. The upload is streamed to disk, checked (MZ
header, no unsafe zip paths) and swapped in; the replaced payload is
archived (two kept) with a Restore button. Install-EngineeringLayer.ps1
stays repo-owned and now installs the newest Engineering_Install_v*.exe.

KB: Part 4 becomes Site Content (enrollment package, Engineering Layer,
Blancco site settings, Shop Floor apps) plus a new Adding Shop Floor
Apps section for preinstall.json/postppkg.json. WJ examples removed.

playbook/engineering/Install-EngineeringLayer.ps1

@@ -41,7 +41,10 @@ param(
     # imaging - the unattend installs PowerShell 7 from C:\PreInstall\installers.
     [string]$StagingRoot = 'C:\PreInstall\engineering',
     [string]$LogDir      = 'C:\Logs\Engineering',
-    [string]$Installer   = 'engineering_layer\Engineering_Install_v3.1.exe',
+    # Relative to StagingRoot. Empty: the newest
+    # engineering_layer\Engineering_Install_v<version>.exe (uploaded on the
+    # PXE webapp, engineer image > Engineering Layer).
+    [string]$Installer   = '',
     [switch]$StageOnly
 )
 
@@ -117,6 +120,18 @@ if (Test-Path -LiteralPath $marker) {
 }
 
 # --- stage the payload out of C:\Deploy -------------------------------------
+function Find-Installer([string]$Dir) {
+    Get-ChildItem -LiteralPath (Join-Path $Dir 'engineering_layer') -Filter 'Engineering_Install_v*.exe' -File -ErrorAction SilentlyContinue |
+        Where-Object { $_.Name -match '^Engineering_Install_v(\d+(\.\d+)*)\.exe$' } |
+        Sort-Object { $v = ($_.Name -replace '^Engineering_Install_v', '') -replace '\.exe$', ''; if ($v -notmatch '\.') { $v += '.0' }; [version]$v } -Descending |
+        Select-Object -First 1
+}
+if (-not $Installer) {
+    $found = Find-Installer $StagingRoot
+    if (-not $found) { $found = Find-Installer $Source }
+    $Installer = if ($found) { 'engineering_layer\' + $found.Name } else { 'engineering_layer\Engineering_Install_v3.1.exe' }
+}
+Write-EngLog "Installer: $Installer"
 $stagedExe = Join-Path $StagingRoot $Installer
 
 if (Test-Path -LiteralPath $stagedExe) {