Add demo_plugin and plugin host integration tests

Minimal in-tree add-on (DP_HELLO) validates registry, ribbon merge, and try_dispatch. Five new lib tests; EntryKind gains PartialEq for test helpers.

src/ui/command_line.rs

@@ -49,7 +49,7 @@ pub struct HistoryEntry {
     pub created_at: Instant,
 }
 
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, PartialEq, Eq)]
 pub enum EntryKind {
     Command,
     Output,