feat(installer): bundle the database - MySQL 8.4 LTS, not 8.0

The bundled-database option could not actually be built. Stage 0 looks for
mysql\mysql-8.0.x-winx64.msi, and Oracle no longer publishes a standalone server
MSI for 8.0 - every 8.0.x returns 404. What remains for 8.0 is the MySQL
Installer bundle, which is an installer-manager: 'msiexec /i INSTALLDIR=' would
install THAT rather than a database, and stage 0 would then fail on a missing
mysqld.exe.

MySQL 8.0 also reached end of life in April 2026, so bundling it would have put
an unsupported database on every new site.

8.4 LTS still ships the standalone MSI (129MB, which is what the '125MB' note in
stage 0 was written against) and is supported into 2032. Defaults follow it:
install root MySQL Server 8.4, service MySQL84. The operator console still looks
for an 8.0 install path as a fallback, for sites already running one.

Also bundles mysqlclient\ - mysql.exe and mysqldump.exe with the two OpenSSL
DLLs they actually import, 20MB rather than the 51MB of debug and auth-plugin
libraries the archive ships. Stage 2 stages it onto the server, so a site whose
database is on ANOTHER host can still take the pre-upgrade backup that every
upgrade depends on. That was the gap the preflight had started warning about.

Bundle is now 221MB.

docs/INSTALL-WINDOWS.md

@@ -35,7 +35,7 @@ time.
 
 Two options. Pick before you start, because they ask different questions.
 
-- **Use the bundled MySQL** - the installer puts MySQL 8.0 on this server and
+- **Use the bundled MySQL** - the installer puts MySQL 8.4 LTS on this server and
   creates the database for you. Choose this when the server has no database
   today. Simplest option, nothing to arrange in advance.
 - **Use an existing MySQL** - the database already exists somewhere, and you have