fix(installer): bundled MySQL install failed on a malformed msiexec command line

Reported from a Windows Server 2019 test: a "Windows Installer" dialog listing
every msiexec /Option appeared, then the wizard reported that the bundled MySQL
database could not be installed. That dialog is msiexec's usage help - it prints
it when the command line does not parse - so the install never started.

Cause: $MysqlRoot defaulted to 'C:\Program Files\MySQL\MySQL Server 8.4', which
contains spaces. Invoke-Native wraps any argument containing whitespace in
quotes, producing "INSTALLDIR=C:\Program Files\...". msiexec takes public
properties as PROPERTY=value and expects the VALUE quoted -
INSTALLDIR="C:\Program Files\..." - so it rejected the line, printed usage, and
exited non-zero.

This file already carried the rule, next to the Python target: "Never put a
space in a path this installer controls." I broke it setting the 8.4 path.

Two fixes, because one of them alone leaves the trap in place:

- $MysqlRoot is now C:\MySQL84, space-free like C:\Python314. The MySQL client
  search paths in the installer, the preflight and the operator console all look
  there first, keeping backups working against the bundled server.
- Invoke-Native now quotes PROPERTY=value correctly, so passing a spaced path
  explicitly no longer produces an unparseable command line.

tests/test_installer_defaults.py fails if an installer-controlled path default
ever contains a space again.

deploy/windows/installer/shopdb-preflight.ps1

@@ -387,7 +387,7 @@ Invoke-Check 'MySQL' 'Backup client' {
     $found = ''
     foreach ($root in @((Join-Path $PSScriptRoot 'mysqlclient'),
                         (Join-Path $AppRoot 'mysqlclient'),
-                        'C:\Program Files\MySQL', 'C:\mysql56\bin',
+                        'C:\MySQL84', 'C:\Program Files\MySQL', 'C:\mysql56\bin',
                         'C:\Program Files (x86)\MySQL')) {
         if (-not (Test-Path $root)) { continue }
         $hit = Get-ChildItem $root -Filter $names[0] -Recurse -ErrorAction SilentlyContinue |