feat(graph): add NewPJRTClient for external PJRT usage

Expose a graph-level factory function that loads a PJRT plugin and creates
a client. This allows external packages (zerfoo) to call CompilePJRT without
directly importing the internal/pjrt package.

Author: dndungu

graph/pjrt_client.go

@@ -0,0 +1,15 @@
+package graph
+
+import "github.com/zerfoo/ztensor/internal/pjrt"
+
+// NewPJRTClient loads a PJRT plugin shared library and creates a client.
+// pluginPath is the bare library filename (e.g. "pjrt_c_api_cpu_plugin.so")
+// or an absolute path. The returned client must be closed when no longer
+// needed (typically after closing all PJRTPlans that reference it).
+func NewPJRTClient(pluginPath string) (*pjrt.Client, error) {
+	lib, err := pjrt.Load(pluginPath)
+	if err != nil {
+		return nil, err
+	}
+	return pjrt.NewClient(lib)
+}