refactor(plugin): drop built-in plugins, external-only

OpenCADStudio no longer bundles plugins — every add-on is an external
cdylib loaded from the plugins folder or installed via the marketplace.

- Remove the demo plugin and the in-tree ocs_example_plugin crate.
- Remove the inventory-based built-in registration path (PluginRegistration,
  all_plugins, installed_manifests); registry now serves core ribbon tabs
  plus loaded external plugins, and try_dispatch routes only to externals.
- Plugin Manager drops the 'compiled into this build' section; the
  enable/disable toggle moves onto loaded external cards.
- Prune the now-dead HostSession/DocumentTab helpers and re-exports.

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

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

src/modules/demo_plugin/dispatch.rs

@@ -1,17 +0,0 @@
-use crate::plugin::host::HostApi;
-
-pub fn handle(host: &mut dyn HostApi, cmd: &str) -> bool {
-    // "DP_IMPORT <path>" arrives from ModuleEvent::PluginFileDialog with the
-    // path in its original case (the command line is bypassed).
-    if let Some(path) = cmd.strip_prefix("DP_IMPORT ") {
-        host.push_info(&format!("demo_plugin imported: {path}"));
-        return true;
-    }
-    match cmd {
-        "DP_HELLO" => {
-            host.push_info("Hello from demo_plugin (plugin host OK).");
-            true
-        }
-        _ => false,
-    }
-}

src/modules/demo_plugin/manifest.rs

@@ -1,14 +0,0 @@
-use crate::plugin::manifest::{ApiVersion, PluginManifest};
-
-pub const PLUGIN_ID: &str = "opencad.demo_plugin";
-
-pub static MANIFEST: PluginManifest = PluginManifest {
-    id: PLUGIN_ID,
-    name: "Demo Plugin",
-    version: "0.1.0",
-    description: "Minimal add-on for plugin-host integration tests",
-    api_version: ApiVersion::CURRENT,
-    ribbon_order: 99,
-    xdata_apps: &[],
-    command_prefixes: &["DP_"],
-};

src/modules/demo_plugin/mod.rs

@@ -1,62 +0,0 @@
-// Minimal in-tree add-on — validates plugin host on `feature/plugin-host`.
-
-pub mod dispatch;
-pub mod manifest;
-pub mod plugin;
-pub mod register;
-
-use crate::modules::{CadModule, IconKind, ModuleEvent, RibbonGroup, RibbonItem, ToolDef};
-
-inventory::submit!(crate::command::CommandRegistration {
-    names: &["DP_HELLO", "DP_IMPORT"]
-});
-
-pub struct DemoPluginModule;
-
-impl CadModule for DemoPluginModule {
-    fn id(&self) -> &'static str {
-        "demo_plugin"
-    }
-
-    fn title(&self) -> &'static str {
-        "Demo Plugin"
-    }
-
-    fn ribbon_groups(&self) -> Vec<RibbonGroup> {
-        vec![RibbonGroup {
-            title: "Smoke",
-            tools: vec![
-                RibbonItem::LargeTool(ToolDef {
-                    id: "DP_HELLO",
-                    label: "Hello",
-                    icon: IconKind::Glyph("★"),
-                    event: ModuleEvent::Command("DP_HELLO".to_string()),
-                }),
-                // Exercises ModuleEvent::PluginFileDialog: the host opens a
-                // native picker and dispatches "DP_IMPORT <path>" back here.
-                RibbonItem::LargeTool(ToolDef {
-                    id: "DP_IMPORT",
-                    label: "Import",
-                    icon: IconKind::Glyph("📂"),
-                    event: ModuleEvent::PluginFileDialog {
-                        command: "DP_IMPORT".to_string(),
-                        title: "Import Demo File".to_string(),
-                        filter_name: "Text".to_string(),
-                        extensions: vec!["txt".to_string(), "csv".to_string()],
-                    },
-                }),
-            ],
-        }]
-    }
-}
-
-#[cfg(test)]
-mod tests {
-    use crate::plugin::all_ribbon_modules;
-
-    #[test]
-    fn ribbon_tab_is_registered() {
-        let titles: Vec<&str> = all_ribbon_modules().iter().map(|m| m.title()).collect();
-        assert!(titles.contains(&"Demo Plugin"), "tabs: {titles:?}");
-    }
-}

src/modules/demo_plugin/plugin.rs

@@ -1,21 +0,0 @@
-use crate::plugin::host::{BuiltinPlugin, HostApi};
-use crate::plugin::manifest::PluginManifest;
-
-use super::dispatch;
-use super::manifest;
-
-pub struct DemoPlugin;
-
-impl BuiltinPlugin for DemoPlugin {
-    fn manifest(&self) -> &'static PluginManifest {
-        &manifest::MANIFEST
-    }
-
-    fn ribbon(&self) -> Box<dyn crate::modules::CadModule> {
-        Box::new(super::DemoPluginModule)
-    }
-
-    fn dispatch(&self, host: &mut dyn HostApi, cmd: &str) -> bool {
-        dispatch::handle(host, cmd)
-    }
-}

src/modules/demo_plugin/plugin.toml

@@ -1,13 +0,0 @@
-[plugin]
-id = "opencad.demo_plugin"
-name = "Demo Plugin"
-version = "0.1.0"
-description = "Minimal add-on for plugin-host integration tests"
-author = "Open CAD Studio contributors"
-license = "GPL-3.0-only"
-
-[opencad]
-api_version = 1
-ribbon_order = 99
-command_prefixes = ["DP_"]
-xdata_apps = []

src/modules/demo_plugin/register.rs

@@ -1,7 +0,0 @@
-use super::plugin::DemoPlugin;
-
-inventory::submit! {
-    crate::plugin::registry::PluginRegistration {
-        construct: || Box::new(DemoPlugin),
-    }
-}

src/modules/mod.rs

@@ -29,7 +29,6 @@ pub mod draw;
 pub mod insert;
 pub mod model;
 pub mod layout;
-pub mod demo_plugin;
 pub mod manage;
 pub mod view;