Fix container preflight checks across Linux and Windows.

Make pandoc preflight succeed when only Quarto-bundled pandoc is available, and harden Windows error logging to avoid PowerShell interpolation failures in catch blocks.

.github/workflows/book-build-container.yml

@@ -403,8 +403,8 @@ jobs:
 
           run_check "quarto on PATH" "command -v quarto >/dev/null"
           run_check "quarto version" "quarto --version | sed -n '1p'"
-          run_check "pandoc on PATH" "command -v pandoc >/dev/null"
+          run_check "pandoc available" "command -v pandoc >/dev/null || quarto pandoc --version >/dev/null 2>&1"
-          run_check "pandoc version" "pandoc --version | sed -n '1p'"
+          run_check "pandoc version" "if command -v pandoc >/dev/null; then pandoc --version | sed -n '1p'; else quarto pandoc --version | sed -n '1p'; fi"
           run_check "python3 on PATH" "command -v python3 >/dev/null"
           run_check "python3 version" "python3 --version"
           run_check "mlsysim import" "python3 -c \"import mlsysim,sys; print('mlsysim:', mlsysim.__file__); print('python:', sys.executable)\""
@@ -446,15 +446,31 @@ jobs:
                 if (-not `$?) { throw 'Command returned non-zero status' }
                 Write-Host \"✅ preflight: `$Name\"
               } catch {
-                Write-Error \"❌ preflight failed: `$Name -- `$(`$_.Exception.Message)\"
+                `$errorMessage = `$_.Exception.Message
+                Write-Error (\"❌ preflight failed: `$Name -- {0}\" -f `$errorMessage)
                 throw
               }
             }
 
             Invoke-Check 'quarto on PATH' { `$resolved = Get-Command quarto -ErrorAction Stop; Write-Host \"   quarto -> `$(`$resolved.Source)\" }
             Invoke-Check 'quarto version' { quarto --version | Select-Object -First 1 }
-            Invoke-Check 'pandoc on PATH' { `$resolved = Get-Command pandoc -ErrorAction Stop; Write-Host \"   pandoc -> `$(`$resolved.Source)\" }
+            Invoke-Check 'pandoc available' {
-            Invoke-Check 'pandoc version' { pandoc --version | Select-Object -First 1 }
+              `$pandocCmd = Get-Command pandoc -ErrorAction SilentlyContinue
+              if (`$pandocCmd) {
+                Write-Host \"   pandoc -> `$(`$pandocCmd.Source)\"
+              } else {
+                quarto pandoc --version | Select-Object -First 1 | Out-Null
+                Write-Host \"   pandoc -> bundled via quarto\"
+              }
+            }
+            Invoke-Check 'pandoc version' {
+              `$pandocCmd = Get-Command pandoc -ErrorAction SilentlyContinue
+              if (`$pandocCmd) {
+                pandoc --version | Select-Object -First 1
+              } else {
+                quarto pandoc --version | Select-Object -First 1
+              }
+            }
             Invoke-Check 'python on PATH' { `$resolved = Get-Command python -ErrorAction Stop; Write-Host \"   python -> `$(`$resolved.Source)\" }
             Invoke-Check 'python version' { python --version }
             Invoke-Check 'python3 on PATH' { `$resolved = Get-Command python3 -ErrorAction Stop; Write-Host \"   python3 -> `$(`$resolved.Source)\" }