feat(plugin): marketplace — install plugins from a curated registry

Add a Plugin Manager marketplace that installs external add-ons from a
GitHub repo's Releases. A curated registry (plugins/registry.json in this
repo, extended by PR) lists discoverable plugins; the host fetches it,
shows each entry with a release dropdown, and on install downloads the
asset matching the user's platform plus plugin.toml into the plugins
folder (API-version checked first). Users can also link arbitrary
owner/repo manually. Linked repos persist in settings.

Desktop only (ureq + serde_json); the install path reuses the phase-2
loader. Part of the #100 extensibility epic (phase 2).

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

plugins/README.md

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+# Plugin registry
+
+`registry.json` is the curated list of third-party plugins Open CAD Studio
+offers in its **Plugin Manager → marketplace**. The app fetches it from this
+repo's `main` branch at runtime, so adding an entry makes a plugin discoverable
+to every user without an app update.
+
+## Add your plugin
+
+Open a pull request adding one object to the array in
+[`registry.json`](registry.json):
+
+```json
+{
+  "repo": "your-account/your-plugin-repo",
+  "name": "Human-readable name",
+  "description": "One line describing what it does."
+}
+```
+
+Requirements for the linked repo:
+
+- Builds a `cdylib` against [`ocs_plugin_api`](../crates/ocs_plugin_api) and
+  exports the host symbols via `ocs_plugin_api::export_plugin!`.
+- Publishes per-platform binaries plus `plugin.toml` as **GitHub Release**
+  assets (see `crates/ocs_example_plugin` and its release workflow for a
+  template). Asset names carry the platform, e.g.
+  `your.plugin-linux-x86_64.so`, `…-windows-x86_64.dll`, `…-macos-aarch64.dylib`.
+- `plugin.toml` declares an `api_version` compatible with the host.
+
+The host reads the release matching the user's platform, checks the API
+version, and installs it into the user's plugins folder.
+
+> Listing is curation, not endorsement or a security review. Installing a
+> plugin runs its native code; users install at their own risk.

plugins/registry.json

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+[
+  {
+    "repo": "HakanSeven12/opencad-example-plugin",
+    "name": "Example Plugin",
+    "description": "Reference dynamically-loaded add-on (ribbon tab + EX_HELLO command)."
+  }
+]