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/plugin.toml

@@ -0,0 +1,13 @@
+[plugin]
+id = "opencad.example"
+name = "Example Plugin"
+version = "0.1.0"
+description = "Reference dynamically-loaded add-on"
+author = "Open CAD Studio contributors"
+license = "GPL-3.0-only"
+
+[opencad]
+api_version = 1
+ribbon_order = 50
+command_prefixes = ["EX_"]
+xdata_apps = []