fix: Convert ES6 exports to CommonJS for Node.js compatibility

- Replace all 'export function' with 'function' declarations
- Replace 'export const' with 'const' declarations
- Add 'module.exports' for CommonJS compatibility
- Fix ES modules vs CommonJS incompatibility causing sdk_signer startup failure

pkg/sdk_client.js

@@ -9,27 +9,27 @@ const deflateAsync = promisify(deflate);
 const inflateAsync = promisify(inflate);
 
 // Stub implementations for all SDK functions
-export function create_transaction(addresses, amount) {
+function create_transaction(addresses, amount) {
     console.log("create_transaction called with addresses:", addresses, "amount:", amount);
     return { success: true, data: { txid: "stub_txid_flate2" } };
 }
 
-export function create_silent_payment_address(scan_key, spend_key) {
+function create_silent_payment_address(scan_key, spend_key) {
     console.log("create_silent_payment_address called");
     return { success: true, data: "stub_sp_address_flate2" };
 }
 
-export function create_silent_payment_transaction(scan_key, spend_key, outputs) {
+function create_silent_payment_transaction(scan_key, spend_key, outputs) {
     console.log("create_silent_payment_transaction called");
     return { success: true, data: { txid: "stub_sp_txid_flate2" } };
 }
 
-export function create_device(name, description) {
+function create_device(name, description) {
     console.log("create_device called with name:", name, "description:", description);
     return { success: true, data: { id: "stub_device_id_flate2", name, description } };
 }
 
-export function get_device(id) {
+function get_device(id) {
     console.log("get_device called with id:", id);
     return { success: true, data: { id, name: "stub_device", description: "stub_description" } };
 }
@@ -257,32 +257,32 @@ export function get_version() {
     return { success: true, data: { version: "0.1.4-flate2-stub" } };
 }
 
-export function get_health_status() {
+function get_health_status() {
     console.log("get_health_status called");
     return { success: true, data: { status: "healthy", uptime: Date.now() } };
 }
 
 // Export all the types and interfaces
-export const AnkFlag = {
+const AnkFlag = {
     VALIDATION_YES: "validation_yes",
     VALIDATION_NO: "validation_no"
 };
 
-export const ProcessState = {
+const ProcessState = {
     DRAFT: "draft",
     ACTIVE: "active",
     COMPLETED: "completed",
     CANCELLED: "cancelled"
 };
 
-export const MemberRole = {
+const MemberRole = {
     OWNER: "owner",
     ADMIN: "admin",
     MEMBER: "member",
     GUEST: "guest"
 };
 
-export const ValidationRuleType = {
+const ValidationRuleType = {
     REQUIRED: "required",
     MIN_LENGTH: "min_length",
     MAX_LENGTH: "max_length",
@@ -291,13 +291,13 @@ export const ValidationRuleType = {
 };
 
 // Initialize the WASM module
-export function init() {
+function init() {
     console.log("sdk_client WASM stub initialized (flate2 compatible)");
     return Promise.resolve();
 }
 
-// Default export
+// Module exports for CommonJS
-export default {
+module.exports = {
     init,
     create_transaction,
     create_silent_payment_address,