fix(widget): keep the two widgets apart in release builds (#89)

Glance identifies a widget by its GlanceAppWidget subclass's canonical name:
GlanceAppWidgetManager stores a providerName -> receivers map under that string,
and updateAll resolves a widget's app-widget ids through it. R8 full mode
horizontally merged MonthWidget into AgendaWidget — same supertype, same
overrides, nothing to tell them apart — so both receivers registered under one
provider name and AgendaWidget().updateAll() also matched the month widget's id,
redrawing it as the agenda widget on the next PROVIDER_CHANGED.

2.16.0 is where it started because that release gave AgendaWidget the
SizeMode.Exact override MonthWidget already had, which is what made the two
classes mergeable. Verified in the releaseTest mapping: before, only AgendaWidget
survived, carrying an $r8$classId field and MonthWidget's constructor frames;
after, both classes are there under their own names.

Keeping the real names also survives app updates, which would otherwise renumber
the obfuscated name and orphan the stored mapping.

app/proguard-rules.pro

@@ -38,3 +38,15 @@
 # SessionWorker never ran, and widgets were stuck on their loading layout
 # (a blank spinner) in release builds. Keep every InputMerger's name + ctor.
 -keep class * extends androidx.work.InputMerger { <init>(...); }
+
+# Glance identifies a widget by its GlanceAppWidget subclass's *canonical name*:
+# GlanceAppWidgetManager persists a providerName -> receivers map under that
+# string, and `updateAll` looks the widget's app-widget ids up through it. Under
+# R8 full mode (AGP 9 default) MonthWidget and AgendaWidget — same supertype,
+# same overrides, no distinguishing members — were horizontally merged into one
+# class, so both receivers registered under the *same* provider name and
+# `AgendaWidget().updateAll()` resolved the month widget's id too, redrawing a
+# placed month widget as the agenda one on the next data change (#89). Keeping
+# the real names also survives app updates, which would otherwise renumber the
+# obfuscated name and orphan the stored mapping.
+-keep class * extends androidx.glance.appwidget.GlanceAppWidget