feat(plugin): lift host surface behind a HostApi trait (phase 1b)

Plugins targeted the host's concrete HostSession type. Add a HostApi
trait in ocs_plugin_api behind an optional `host` feature (the only thing
that pulls acadrust, so the core crate stays dependency-free). HostSession
implements it and BuiltinPlugin::dispatch now takes `&mut dyn HostApi`, so
an out-of-tree add-on compiles against the contract crate alone. Per-tab
plugin state is reached through object-safe plugin_state* helpers.

Completes the phase-1 host-surface extraction in the #100 epic.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/plugin-template/plugin.rs

@@ -1,4 +1,4 @@
-use crate::plugin::host::{BuiltinPlugin, HostSession};
+use crate::plugin::host::{BuiltinPlugin, HostApi};
 use crate::plugin::manifest::PluginManifest;
 
 use super::dispatch;
@@ -15,7 +15,7 @@ impl BuiltinPlugin for MyPlugin {
         Box::new(super::MyPluginModule)
     }
 
-    fn dispatch(&self, host: &mut HostSession<'_>, cmd: &str) -> bool {
+    fn dispatch(&self, host: &mut dyn HostApi, cmd: &str) -> bool {
         dispatch::handle(host, cmd)
     }
 }