feat(plugin): ModuleEvent::PluginFileDialog for native file import

Add-ons had no way to request a file picker — only the host's own Open
dialog. PluginFileDialog lets a plugin tool ask the host to open a native
picker; on selection the host dispatches "<command> <path>" back to the
plugin with original case preserved (bypassing the command-line
upper-casing that mangles case-sensitive paths on Linux/macOS). The demo
plugin gains an Import tool exercising it.

Part of the #100 extensibility epic (surfaced in #106).

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

src/modules/demo_plugin/dispatch.rs

@@ -1,6 +1,12 @@
 use crate::plugin::host::HostSession;
 
 pub fn handle(host: &mut HostSession<'_>, 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).");

src/modules/demo_plugin/mod.rs

@@ -8,7 +8,7 @@ pub mod register;
 use crate::modules::{CadModule, IconKind, ModuleEvent, RibbonGroup, RibbonItem, ToolDef};
 
 inventory::submit!(crate::command::CommandRegistration {
-    names: &["DP_HELLO"]
+    names: &["DP_HELLO", "DP_IMPORT"]
 });
 
 pub struct DemoPluginModule;
@@ -25,12 +25,27 @@ impl CadModule for DemoPluginModule {
     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()),
-            })],
+            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()],
+                    },
+                }),
+            ],
         }]
     }
 }