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/Cargo.toml

@@ -6,3 +6,12 @@ description = "Stable, dependency-free contract for Open CAD Studio add-on packa
 license = "GPL-3.0-only"
 
 [dependencies]
+# Pulled in only by the `host` feature, which adds the `acadrust`-typed
+# `HostApi` runtime surface. The default crate stays dependency-free so engine
+# crates and external tooling can depend on the manifest/ribbon contract cheaply.
+acadrust = { version = "0.3.4", optional = true }
+
+[features]
+# Enables the runtime host surface (`HostApi` trait). The OpenCADStudio binary
+# turns this on; pure-data consumers leave it off.
+host = ["dep:acadrust"]