manifest engine: null-safe PS1 dispatch (accept Script or Installer, log resolved path)

PS1 entries were crashing with a cryptic "Cannot bind LiteralPath because it is
null" when the resolved script path came back null - the per-entry try/catch
caught it (so the scope survived) but the cause was opaque. Now the PS1 branch
accepts either Script or Installer, null-guards before Join-Path/Test-Path, and
logs the resolved relative path, so a bad/empty entry is skipped with a clear
"has no Script/Installer value" line instead of a null-bind throw.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

playbook/shopfloor-setup/common/lib/Install-FromManifest.ps1

@@ -354,9 +354,18 @@ function Invoke-InstallerAction {
             return [pscustomobject]@{ ExitCode = $proc.ExitCode; LogRef = $App.LogFile }
         }
         'PS1' {
-            $scriptPath = Join-Path $InstallerRoot ($App.Script)
+            # Accept either Script or Installer as the relative path, and never
+            # feed a null into Join-Path/Test-Path (that throws a cryptic
+            # 'LiteralPath is null'). Log the resolved value so a bad/empty
+            # entry is obvious in the log instead of crashing the entry.
+            $rel = if ($App.Script) { $App.Script } elseif ($App.Installer) { $App.Installer } else { $null }
+            if ([string]::IsNullOrWhiteSpace([string]$rel)) {
+                Write-InstallLog ("  PS1 entry '{0}' has no Script/Installer value (Script={1}, Installer={2}) - skipping" -f $App.Name, $App.Script, $App.Installer) 'ERROR'
+                return [pscustomobject]@{ ExitCode = -1; LogRef = $null }
+            }
+            $scriptPath = Join-Path $InstallerRoot $rel
             if (-not (Test-Path -LiteralPath $scriptPath)) {
-                Write-InstallLog "  PS1 not found: $scriptPath" 'ERROR'
+                Write-InstallLog "  PS1 not found: $scriptPath (from rel '$rel')" 'ERROR'
                 return [pscustomobject]@{ ExitCode = -1; LogRef = $null }
             }
             $psi.FileName  = 'powershell.exe'