Fix duplicate answer processing race condition (#6)

Add polling guard to prevent concurrent pollInternal() execution.
The setInterval callback doesn't await the async pollInternal(),
which could cause multiple polls to process the same answer before
lastPollTimestamp is updated, resulting in "Called in wrong state:
stable" errors from setRemoteDescription().

src/rondevu.ts

@@ -259,6 +259,7 @@ export class Rondevu extends EventEmitter {
     private filling = false
     private pollingInterval: ReturnType<typeof setInterval> | null = null
     private lastPollTimestamp = 0
+    private isPolling = false  // Guard against concurrent poll execution
 
     private constructor(
         apiUrl: string,
@@ -634,6 +635,13 @@ export class Rondevu extends EventEmitter {
     private async pollInternal(): Promise<void> {
         if (!this.filling) return
 
+        // Prevent concurrent poll execution to avoid duplicate answer processing
+        if (this.isPolling) {
+            this.debug('Poll already in progress, skipping')
+            return
+        }
+
+        this.isPolling = true
         try {
             const result = await this.api.poll(this.lastPollTimestamp)
 
@@ -674,6 +682,8 @@ export class Rondevu extends EventEmitter {
             }
         } catch (err) {
             console.error('[Rondevu] Polling error:', err)
+        } finally {
+            this.isPolling = false
         }
     }
 
@@ -710,6 +720,7 @@ export class Rondevu extends EventEmitter {
     stopFilling(): void {
         this.debug('Stopping offer filling and polling')
         this.filling = false
+        this.isPolling = false  // Reset polling guard
 
         // Stop polling
         if (this.pollingInterval) {