feat(plugin): discover external plugin packages from the plugins folder

Phase-2 groundwork: scan <config>/OpenCADStudio/plugins/<id>/plugin.toml,
parse the documented keys (no new TOML dep), check for a platform native
library beside it, and gate on the API version. The Plugin Manager gains
an "External (plugins folder)" section showing each package with a
status pill (Ready to load / API incompatible / No library). Discovery
only — actually loading the cdylib is the next step.

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

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/plugin/mod.rs

@@ -4,6 +4,7 @@
 // Domain plugins (e.g. storm_sewer) live under `src/modules/<name>/` and
 // register here via `inventory::submit!(PluginRegistration { … })`.
 
+pub mod external;
 pub mod host;
 pub mod manifest;
 pub mod registry;