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/dispatch.rs

@@ -1,8 +1,8 @@
-use crate::plugin::host::HostSession;
+use crate::plugin::host::HostApi;
 
 use super::manifest::PLUGIN_ID;
 
-pub fn handle(host: &mut HostSession<'_>, cmd: &str) -> bool {
+pub fn handle(host: &mut dyn HostApi, cmd: &str) -> bool {
     let _ = (host, PLUGIN_ID);
     match cmd {
         "MP_HELLO" => {