fix: acknowledge pi maintenance barrier

backend/src/runtime/maintenance-gate.ts

@@ -1,10 +1,27 @@
-import { existsSync } from "node:fs";
+/** An in-process admission barrier. A lease spans the complete create/resume decision. */
+export class MaintenanceBarrier {
+  private active = false;
+  private admissions = 0;
+  private waiters: (() => void)[] = [];
 
-/**
- * A host-side lifecycle transaction creates this marker before it checks for active sessions.
- * The marker is intentionally only an admission gate: it must never terminate existing Pi
- * processes or make their in-flight work unavailable.
- */
-export function createMaintenanceGate(markerFile: string): () => boolean {
-  return () => existsSync(markerFile);
+  acquire(): (() => void) | undefined {
+    if (this.active) return undefined;
+    this.admissions += 1;
+    let released = false;
+    return () => {
+      if (released) return;
+      released = true;
+      this.admissions -= 1;
+      if (this.admissions === 0) this.waiters.splice(0).forEach((resolve) => resolve());
+    };
+  }
+
+  async activate(): Promise<void> {
+    this.active = true;
+    if (this.admissions === 0) return;
+    await new Promise<void>((resolve) => this.waiters.push(resolve));
+  }
+
+  deactivate(): void { this.active = false; }
+  status(): { active: boolean; admissions: number } { return { active: this.active, admissions: this.admissions }; }
 }