fix(build): track driver .exe files (gitignore *.exe dropped netkvmp.exe -> DISM rejected NetKVM)

The global .gitignore '*.exe' rule silently excluded netkvmp.exe + netkvmco.exe when the
NetKVM driver was committed, so only inf/sys/cat shipped. netkvm.inf REQUIRES netkvmp.exe
([SourceDisksFiles] + netkvmp.CopyFiles), so Add-WindowsDriver failed every build with 'the
driver package could not be installed' -> no virtio NIC driver -> no VM network. Un-ignore
windows/drivers/** and force-add the referenced binaries.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -68,3 +68,7 @@ coverage/
 
 # SBOM intermediates (final SBOMs are committed; intermediates are not)
 sbom/work/
+
+# Driver binaries (e.g. virtio NetKVM netkvmp.exe) must be tracked despite the global
+# *.exe / *.msi ignores above -- they are referenced by the .inf and DISM needs them.
+!windows/drivers/**