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>

Cargo.lock

@@ -27,6 +27,7 @@ dependencies = [
  "image",
  "inventory",
  "js-sys",
+ "libloading",
  "lzma-sys",
  "ocs_plugin_api",
  "open",
@@ -3623,6 +3624,13 @@ dependencies = [
  "objc2-foundation 0.2.2",
 ]
 
+[[package]]
+name = "ocs_example_plugin"
+version = "0.1.0"
+dependencies = [
+ "ocs_plugin_api",
+]
+
 [[package]]
 name = "ocs_plugin_api"
 version = "0.1.0"