refactor(plugin): extract ocs_plugin_api crate for the stable add-on contract

Move the dependency-free, semver-versioned half of the plugin contract into a
standalone workspace crate `crates/ocs_plugin_api`:

- manifest types: PluginManifest, ApiVersion, API_VERSION
- ribbon vocabulary: CadModule trait + ToolDef/RibbonGroup/RibbonItem/IconKind/
  ModuleEvent/StyleKey

The host re-exports them from `crate::plugin::manifest` and `crate::modules`, so
every existing call site is unchanged. The acadrust-typed runtime surface
(HostSession) stays in the host binary; lifting it behind a HostApi trait in the
same crate is the remaining phase-1b step.

Part of #100. Docs updated in docs/plugin-architecture.md.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/plugin/manifest.rs

@@ -1,32 +1,7 @@
-// Plugin identity and capability declaration.
+//! Plugin identity and capability declaration.
+//!
+//! These types now live in the standalone, dependency-free `ocs_plugin_api`
+//! crate so external add-ons can target a semver-stable contract. Re-exported
+//! here to keep the `crate::plugin::manifest::*` path stable for in-tree use.
 
-/// Host plugin API version. Bump when HostApi breaks compatibility.
-pub const API_VERSION: u32 = 1;
-
-#[derive(Clone, Copy, Debug, PartialEq, Eq)]
-pub struct ApiVersion {
-    pub major: u32,
-}
-
-impl ApiVersion {
-    pub const CURRENT: Self = Self { major: API_VERSION };
-
-    pub fn is_compatible_with(host: ApiVersion) -> bool {
-        Self::CURRENT.major == host.major
-    }
-}
-
-/// Static metadata every plugin supplies at registration time.
-/// Keep fields in sync with `plugin.toml` beside the package.
-#[derive(Clone, Copy, Debug)]
-pub struct PluginManifest {
-    pub id: &'static str,
-    pub name: &'static str,
-    pub version: &'static str,
-    pub description: &'static str,
-    pub api_version: ApiVersion,
-    /// Sort key for add-on ribbon tabs (lower = further left among plugins).
-    pub ribbon_order: i32,
-    pub xdata_apps: &'static [&'static str],
-    pub command_prefixes: &'static [&'static str],
-}
+pub use ocs_plugin_api::manifest::*;