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.toml

@@ -4,7 +4,13 @@ version = "0.5.6"
 edition = "2021"
 build = "build.rs"
 
+[workspace]
+members = ["crates/ocs_plugin_api"]
+
 [dependencies]
+# Stable, dependency-free add-on contract (manifest + ribbon/CadModule types).
+# Plugin authors target this crate's semver, not OpenCADStudio internals.
+ocs_plugin_api = { path = "crates/ocs_plugin_api" }
 # vtkio (pulled in transitively via truck-meshalgo) depends on xz2 → lzma-sys,
 # which by default links the system liblzma dynamically. On macOS that bakes a
 # Homebrew dylib path (/opt/homebrew/.../liblzma.5.dylib) into the binary, so