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>
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