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/modules/demo_plugin/dispatch.rs

@@ -0,0 +1,11 @@
+use crate::plugin::host::HostSession;
+
+pub fn handle(host: &mut HostSession<'_>, cmd: &str) -> bool {
+    match cmd {
+        "DP_HELLO" => {
+            host.push_info("Hello from demo_plugin (plugin host OK).");
+            true
+        }
+        _ => false,
+    }
+}