feat: add C++ support with ImGui/ImPlot framework and vendor submodules

Añade soporte C++ al registry: vendor submodules (glfw, imgui, implot, tracy),
sistema de build con CMake y toolchains cross-platform, runner C++ en fn CLI,
parser de tests Google Test, y funciones bash para build Linux/Windows.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cmd/fn/run.go

@@ -103,6 +103,8 @@ func buildCommand(fn *registry.Function, db *registry.DB, registryRoot, absPath
 		return buildBashCommand(absPath, args)
 	case "ts":
 		return buildTsCommand(registryRoot, absPath, args)
+	case "cpp":
+		return buildCppCommand(fn, registryRoot, absPath, args)
 	default:
 		return nil, fmt.Errorf("unsupported lang %q for execution", fn.Lang)
 	}