feat(plugin): wrap external-plugin calls in a panic guard

A buggy external plugin that panicked anywhere — during load (version,
register, manifest), during ribbon build / dispatch, or inside an
interactive command (prompt, on_point, on_enter, on_object_pick) — took
the host down with it (#134). Run every cdylib entry point under
catch_unwind so the host stays up: the plugin's call is treated as
inert (load fails, dispatch returns "didn't handle it", interactive
hooks resolve to Cancel) and a "Plugin <hook> panicked: …" line lands
in the command line on the next tick. (#145)

Scope is Rust panics across the cdylib boundary, not genuine UB —
real isolation (segfault-proof) still requires out-of-process plugins.

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

src/plugin/external.rs

@@ -311,7 +311,10 @@ mod loader {
             let version: libloading::Symbol<extern "C" fn() -> u32> = lib
                 .get(b"ocs_plugin_api_version")
                 .map_err(|_| "missing ocs_plugin_api_version symbol".to_string())?;
-            let v = version();
+            // Wrap the version + register calls in a panic guard so a plugin
+            // that panics during load can't take down the host. (#145)
+            let v = crate::plugin::guard("version", || version())
+                .ok_or_else(|| "ocs_plugin_api_version panicked".to_string())?;
             if v != ocs_plugin_api::API_VERSION {
                 return Err(format!(
                     "API version {v} != host {}",
@@ -324,12 +327,16 @@ mod loader {
             > = lib
                 .get(b"ocs_plugin_register")
                 .map_err(|_| "missing ocs_plugin_register symbol".to_string())?;
-            let raw = register();
+            let raw = crate::plugin::guard("register", || register())
+                .ok_or_else(|| "ocs_plugin_register panicked".to_string())?;
             if raw.is_null() {
                 return Err("ocs_plugin_register returned null".into());
             }
             let plugin = *Box::from_raw(raw);
-            let id = plugin.manifest().id.to_string();
+            // The manifest read happens once at load; guard it too — a buggy
+            // manifest() that panics here would otherwise crash startup. (#145)
+            let id = crate::plugin::guard("manifest", || plugin.manifest().id.to_string())
+                .ok_or_else(|| "plugin manifest() panicked".to_string())?;
             Ok(LoadedPlugin {
                 plugin,
                 _lib: lib,