Write through the mapped drive, record the UNC

The snapshot path stored against a marker read "W:\backup\partmarker\...". That
mapping exists only inside the SYSTEM logon session, only while the GE-Enforce
cycle is running, and only on that one bay - so the path recorded in ShopDB
resolved to nothing for the person most likely to need it.

Writing still goes through the drive letter, deliberately: that is the
connection GE-Enforce authenticated, and reaching the same server by another
name costs a second authentication or fails outright with error 1219. Only the
recorded path changes.

Resolving a mapped drive to its share needs all three of DisplayRoot, Root and
the WMI ProviderName, because which one is populated depends on how the drive
was made: net use mappings (what GE-Enforce creates) fill DisplayRoot, while a
New-PSDrive to a UNC leaves DisplayRoot EMPTY and puts the share in Root. A
local drive and an already-UNC path both fall through untouched.

playbook/shopfloor-setup/common/scripts/Backup-PartMarker.ps1

@@ -179,6 +179,61 @@ function Get-SnapshotManifest {
 }
 
 
+function ConvertTo-UncPath {
+    <#
+    .SYNOPSIS
+        The UNC equivalent of a path on a mapped drive, or the path unchanged.
+    .DESCRIPTION
+        We WRITE through the drive letter GE-Enforce mapped, because that is the
+        connection it authenticated - reaching the same server by another name
+        can cost a second authentication or fail outright with error 1219.
+
+        But the path RECORDED in ShopDB has to be one a human can use. "W:" is a
+        mapping that exists only inside the SYSTEM session, only while the cycle
+        runs, and only on that one PC. Storing it means a tech opening the
+        marker's page gets a path that resolves to nothing anywhere.
+
+        So: write by drive, record by UNC. DisplayRoot on a mapped PSDrive is
+        the share it points at; a local drive has none, and a path that is
+        already UNC needs no help - both fall through unchanged.
+    #>
+    param([Parameter(Mandatory)][string]$Path)
+
+    if ($Path -like '\\*') { return $Path }
+
+    $qualifier = ''
+    try { $qualifier = Split-Path -Qualifier $Path } catch { return $Path }
+    if (-not $qualifier) { return $Path }
+
+    $letter = $qualifier.TrimEnd(':')
+    $rest = Split-Path -NoQualifier $Path
+
+    # Three ways a drive can tell you where it points, and which one is
+    # populated depends on how it was created:
+    #   DisplayRoot  set by `net use` mappings - what GE-Enforce makes
+    #   Root         set by New-PSDrive to a UNC, where DisplayRoot is EMPTY
+    #   ProviderName the WMI view, as a backstop if the PSDrive lookup is odd
+    try {
+        $drive = Get-PSDrive -Name $letter -ErrorAction Stop
+        foreach ($candidate in @($drive.DisplayRoot, $drive.Root)) {
+            if ($candidate -and $candidate -like '\\*') {
+                return ($candidate.TrimEnd('\') + $rest)
+            }
+        }
+    } catch { }
+
+    try {
+        $disk = Get-CimInstance -ClassName Win32_LogicalDisk `
+                    -Filter ("DeviceID='{0}:'" -f $letter) -ErrorAction Stop
+        if ($disk.ProviderName -and $disk.ProviderName -like '\\*') {
+            return ($disk.ProviderName.TrimEnd('\') + $rest)
+        }
+    } catch { }
+
+    return $Path
+}
+
+
 # =============================================================================
 # Main
 # =============================================================================
@@ -485,8 +540,16 @@ foreach ($property in (ConvertFrom-Json $manifestjson).PSObject.Properties) {
     $manifesthash[$property.Name] = $property.Value
 }
 
+# Written through $snapshotdir (the mapped drive), recorded as UNC so the path
+# stored against the marker means something on any PC, at any time.
+$reportedpath = ConvertTo-UncPath -Path $snapshotdir
+if ($reportedpath -ne $snapshotdir) {
+    Write-ShopdbBackupLog -Kind $KIND -Message (
+        'Recording {0} (written through {1})' -f $reportedpath, $snapshotdir)
+}
+
 $posted = Send-ShopdbBackup -Context $ctx `
-    -ContentHash $manifest.ManifestHash -SharePath $snapshotdir `
+    -ContentHash $manifest.ManifestHash -SharePath $reportedpath `
     -SourceFileName 'manifest.json' -Manifest $manifesthash -TimeoutSec $TimeoutSec
 
 # Send-ShopdbBackup logs created vs noop. On a no-op the directory just written