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 <noreply@anthropic.com>

crates/ocs_plugin_api/src/manifest.rs

@@ -0,0 +1,51 @@
+//! Plugin identity and capability declaration.
+
+/// Host plugin API version. Bump when the host runtime surface 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
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn current_matches_const() {
+        assert_eq!(ApiVersion::CURRENT.major, API_VERSION);
+    }
+
+    #[test]
+    fn same_major_is_compatible() {
+        assert!(ApiVersion::is_compatible_with(ApiVersion::CURRENT));
+        assert!(!ApiVersion::is_compatible_with(ApiVersion {
+            major: API_VERSION + 1,
+        }));
+    }
+}
+
+/// 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],
+}