feat(client): add keypair generation

plugin/core/state.ts

@@ -1,5 +1,6 @@
 import { mkdir, readFile, rename, writeFile } from "node:fs/promises";
 import { dirname } from "node:path";
+import { generateKeyPair, type KeyPair } from "../crypto/keypair.js";
 
 export const CLIENT_STATE_VERSION = 1;
 
@@ -132,6 +133,31 @@ export function hasClientKeyPair(state: YonexusClientState): boolean {
   );
 }
 
+/**
+ * Ensure the client state has a valid key pair.
+ * If no key pair exists, generates a new Ed25519 key pair.
+ * Returns the (possibly updated) state and whether a new key was generated.
+ */
+export async function ensureClientKeyPair(
+  state: YonexusClientState,
+  stateStore: YonexusClientStateStore
+): Promise<{ state: YonexusClientState; generated: boolean }> {
+  if (hasClientKeyPair(state)) {
+    return { state, generated: false };
+  }
+
+  const keyPair = await generateKeyPair();
+  const updatedState: YonexusClientState = {
+    ...state,
+    privateKey: keyPair.privateKey,
+    publicKey: keyPair.publicKey,
+    updatedAt: Math.floor(Date.now() / 1000)
+  };
+
+  await stateStore.save(updatedState);
+  return { state: updatedState, generated: true };
+}
+
 function assertClientStateShape(
   value: unknown,
   filePath: string