fix: comprehensive dispatch fixes

- go_core_dispatch case 1: tool handler now calls back to plugin via invokeTool
- go_core_dispatch case 2: stage handler sends full StageContext (11 fields)
- New dispatch methods 26-31: Settings.GetCore/SetCore/ListCore/GetPlugin/SetPlugin/ListPlugin
- Remove dlclose (plugin goroutines may still be running after Stop)
- invokeTool/invokeOutput/invokeStage callbacks with proper error handling
- Fix n2 unused variable

internal/plugin/dynamic.go

@@ -72,8 +72,9 @@ func (p *cabiPlugin) Start(s *sdk.PluginSDK) error {
 
 func (p *cabiPlugin) Stop() error {
 	_ = p.handle.Stop()
-	p.handle.FreeCoreAPI()
-	p.handle.Close()
+	// Note: intentionally NOT calling p.handle.Close() (dlclose).
+	// The .so stays loaded because plugin goroutines (HTTP server, tickers)
+	// may still be running. dlclose would unmap their code and cause SIGSEGV.
 	return nil
 }