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

lxs.ps1

@@ -1,6 +1,12 @@
 <#
     LXS - platform dispatcher (Windows side)
 
+    NOTE: this file must stay BOM-less and pure ASCII. It is the one script
+    fetched with `irm ... | iex`, and Invoke-Expression fails on a leading
+    U+FEFF. Being ASCII keeps it parseable when Windows PowerShell 5.1 falls
+    back to the ANSI codepage for a BOM-less file. Everything under windows\
+    carries a UTF-8 BOM instead, because 5.1 reads those from disk.
+
     Thin shim kept at the repo root so the documented one-liner stays short and
     stable while the real implementation lives in windows\lxs.ps1:
 
@@ -40,7 +46,7 @@ if ($LxsSelfDir) {
     }
 }
 
-# No files on disk — we were piped from irm. Fetch the platform entrypoint.
+# No files on disk - we were piped from irm. Fetch the platform entrypoint.
 $temp = Join-Path $env:TEMP ("lxs.entrypoint.{0}.ps1" -f [guid]::NewGuid().ToString('N').Substring(0, 8))
 try {
     Invoke-WebRequest -Uri "$LxsRawBase/$LxsPlatformDir/lxs.ps1" -OutFile $temp `