feat(msi): give a template its own cabinet for per-customer files

Rebranding recompressed the whole ~100MB payload because one cabinet held
everything. In template mode preprocess.py puts the handful of files a custom
client replaces on a second cabinet, so a patch rebuilds a few hundred KB and
leaves the payload cabinet alone. The shipped msi is built without template
mode and keeps its single cabinet.

The branding assets need conditional components. A stock build ships none of
them -- there is no icon.ico, icon.png or logo*.png, only icon.svg -- so the
template has to carry placeholders for the File rows to exist, and a customer
supplies whichever they want. Installing a placeholder unconditionally would
give a customer with no logo a placeholder image, where today a missing asset
means no logo at all: the client tries each candidate and treats the failure as
absence. So each optional asset installs only when its property says the
customer supplied one.

CI creates those placeholders and builds the template with the new mode.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Q7fBdTwziR5BHTkSz7Tzcm

res/msi/Package/Package.wxs

@@ -13,6 +13,12 @@
 		<PropertyRef Id="AddRemovePropertiesFile" />
 
 		<Media Id="1" Cabinet="cab1.cab" EmbedCab="yes" CompressionLevel="high" />
+		<!--$Media2Start$-->
+		<!-- preprocess.py in template mode adds a second cabinet here, holding only
+		     the files that differ per customer, so a custom client can be produced by
+		     rebuilding that small cabinet instead of the whole package. The shipped
+		     msi is built without template mode and keeps a single cabinet. -->
+		<!--$Media2End$-->
 		<Icon Id="AppIcon" SourceFile="Resources\icon.ico" />
 
 		<!-- User Interface -->