fix: enforce one Pi runtime per user

backend/src/pi/pi-process-manager.ts

@@ -12,6 +12,7 @@ export interface SessionRuntime {
   rpc: RpcClient;
   bridge: SessionBridge;
   child: ChildProcessWithoutNullStreams;
+  ownerKey?: string;
 }
 
 export interface RuntimeOptions {
@@ -102,6 +103,15 @@ export class PiProcessManager {
 
   get(id: string): SessionRuntime | undefined { return this.runtimes.get(id); }
 
+  teardownForPrincipal(principal: PrincipalContext): string[] {
+    const ownerKey = `${principal.issuer}\0${principal.subject}`;
+    const stopped: string[] = [];
+    for (const [id, runtime] of [...this.runtimes.entries()]) {
+      if (runtime.ownerKey === ownerKey && this.teardownIfCurrent(id, runtime)) stopped.push(id);
+    }
+    return stopped;
+  }
+
   /** Spawn and register a runtime synchronously, without starting a model turn. */
   createFor(sessionId: string, o: RuntimeOptions = {}): SessionRuntime {
     // A duplicate start must never tear down a live session: that used to send
@@ -110,6 +120,7 @@ export class PiProcessManager {
     if (existing) {
       throw new Error(`session runtime already active: ${sessionId}`);
     }
+    if (o.principal) this.teardownForPrincipal(o.principal);
     if (this.runtimes.size >= this.cfg.maxPiProcesses) {
       throw new Error("max Pi processes reached");
     }
@@ -120,7 +131,12 @@ export class PiProcessManager {
     try {
       const rpc = new RpcClient(child);
       const bridge = new SessionBridge(rpc);
-      const runtime: SessionRuntime = { rpc, bridge, child };
+      const runtime: SessionRuntime = {
+        rpc,
+        bridge,
+        child,
+        ownerKey: o.principal ? `${o.principal.issuer}\0${o.principal.subject}` : undefined,
+      };
       rt = runtime;
       bridge.beginTurn();
       this.runtimes.set(sessionId, runtime);