feat(plugin): load external cdylib plugins at runtime (phase 2)

Move BuiltinPlugin into ocs_plugin_api (host feature) so out-of-tree
crates can implement it, and add export_plugin! to emit the two C
symbols a cdylib exposes: ocs_plugin_api_version (checked first, so an
ABI-incompatible build never runs) and ocs_plugin_register -> boxed
BuiltinPlugin. The host loads every compatible package from the plugins
folder at startup via libloading (desktop only), keeps the library
resident for the session, and merges its ribbon tab + command dispatch
into the same paths as built-ins (honouring enable/disable). The Plugin
Manager shows external packages with a Loaded / incompatible status.

Approach B: the plugin hands back a boxed trait object, assuming a
matching toolchain + ocs_plugin_api version (the version symbol enforces
the latter). crates/ocs_example_plugin is the reference cdylib.

Part of the #100 extensibility epic (phase 2).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/ocs_example_plugin/README.md

@@ -0,0 +1,37 @@
+# ocs_example_plugin
+
+Reference **dynamically-loaded** add-on for Open CAD Studio. It depends only on
+`ocs_plugin_api` (with the `host` feature) — never on the `OpenCADStudio`
+binary — so it shows the full surface an out-of-tree plugin targets: a
+`PluginManifest`, a `CadModule` ribbon tab, a `BuiltinPlugin` entry point, and
+the `export_plugin!` C-ABI export.
+
+## Build & install
+
+```sh
+cargo build -p ocs_example_plugin            # → target/debug/libocs_example_plugin.so
+```
+
+Copy the library and `plugin.toml` into a folder named after the plugin id under
+the user plugins directory:
+
+```
+<config>/OpenCADStudio/plugins/opencad.example/
+  plugin.toml
+  libocs_example_plugin.so      # .dll on Windows, .dylib on macOS
+```
+
+`<config>` is `%APPDATA%` (Windows), `~/Library/Application Support` (macOS), or
+`$XDG_CONFIG_HOME` / `~/.config` (Linux).
+
+Restart Open CAD Studio. The host loads the cdylib at startup (after checking
+`ocs_plugin_api_version`), adds the **Example** ribbon tab, and routes `EX_`
+commands to it. `PLUGINS` lists it under *External* as **Loaded**; run `EX_HELLO`
+to see it respond.
+
+## Contract
+
+- `ocs_plugin_api::export_plugin!(MyPlugin)` emits `ocs_plugin_api_version()` and
+  `ocs_plugin_register()`.
+- The package must be built with the **same toolchain and `ocs_plugin_api`
+  version** as the host (approach B — the version symbol enforces the latter).