refactor(windows scripts): remove BOM and improve UTF-8 handling

windows/tools/repair.ps1

@@ -1,4 +1,4 @@
-<#
+<#
     LXS - System Repair (Windows)
     Description: Integrity repair for the component store and system files
                  (SFC, DISM, chkdsk). Requires administrator rights.
@@ -20,7 +20,10 @@ if ($LxsLibPath -and (Test-Path $LxsLibPath)) {
         Write-Error 'Failed to fetch lib/common.ps1'
         exit 1
     }
-    . ([scriptblock]::Create($LxsLibSource))
+    # The library ships with a UTF-8 BOM (Windows PowerShell 5.1 needs it to
+    # read the file as UTF-8); over HTTP that BOM arrives as a leading U+FEFF
+    # character, which the parser will not accept.
+    . ([scriptblock]::Create(($LxsLibSource -replace '^\uFEFF', '')))
 }
 
 $env:LXS_LOG_FILE = Join-Path (Get-LxsTempDir) 'lxs_repair.log'