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>

Cargo.lock

@@ -24,6 +24,7 @@ dependencies = [
  "image",
  "inventory",
  "lzma-sys",
+ "ocs_plugin_api",
  "open",
  "printpdf",
  "rayon",
@@ -3646,6 +3647,10 @@ dependencies = [
  "objc2-foundation 0.2.2",
 ]
 
+[[package]]
+name = "ocs_plugin_api"
+version = "0.1.0"
+
 [[package]]
 name = "once_cell"
 version = "1.21.4"