Views, events as messages, and a patch, over plain Avalonia

A program is a model, an update and a view. A view is a Bjolang value; an
event finds its message in the view its control was last given, and the next
view is compared with the last so only what differs is changed. Controls are
made with plain constructors and changed with SetValue, so nothing is
subclassed, and a program never names an Avalonia type.

Text, button, text input, checkbox, column and row. tests/app.bjo drives an
app through its real controls with no window.

🤖 Generated with [ECA](https://eca.dev) (anthropic/claude-opus-4-8)

Co-Authored-By: eca-agent <git@eca.dev>

.gitignore

@@ -0,0 +1,8 @@
+.bjo/
+publish/
+*.dll
+*.exe
+*.pdb
+*.bjobuild
+*.runtimeconfig.json
+*.deps.json