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>

crates/ocs_plugin_api/src/lib.rs

@@ -19,6 +19,10 @@
 pub mod manifest;
 pub mod ribbon;
 
+/// Runtime host surface — only built with the `host` feature (pulls `acadrust`).
+#[cfg(feature = "host")]
+pub mod host;
+
 pub use manifest::{ApiVersion, PluginManifest, API_VERSION};
 pub use ribbon::{
     CadModule, IconKind, ModuleEvent, RibbonGroup, RibbonItem, StyleKey, ToolDef,