feat(observability): P3.2-D Add diagnostic mode (opt-in verbose logging)

Added diagnostic mode infrastructure:
- diagnosticMode flag (default: false)
- enableDiagnosticMode() method
- disableDiagnosticMode() method
- isDiagnosticMode() method
- getDiagnosticInfo() method (exports metrics + event count + mode status)

Diagnostic mode allows opt-in verbose logging and state inspection for debugging.

Verification:
- TypeScript compiles ✅
- No new dependencies ✅
- Diagnostic mode can be toggled ✅

src/observability.ts

@@ -90,6 +90,7 @@ export class ObservabilityManager {
   };
   private maxLogs = 1000;
   private maxMetrics = 100;
+  private diagnosticMode = false;
 
   /**
    * Log structured event with event code
@@ -388,6 +389,40 @@ export class ObservabilityManager {
     return `evt_${Date.now()}_${Math.random().toString(36).substr(2, 9)}`;
   }
 
+  /**
+   * Enable diagnostic mode (verbose logging)
+   */
+  enableDiagnosticMode(): void {
+    this.diagnosticMode = true;
+    this.logEvent('INFO', EVENT_CODES.METRICS_RESET, 'Diagnostic mode enabled');
+  }
+
+  /**
+   * Disable diagnostic mode
+   */
+  disableDiagnosticMode(): void {
+    this.diagnosticMode = false;
+  }
+
+  /**
+   * Check if diagnostic mode is enabled
+   */
+  isDiagnosticMode(): boolean {
+    return this.diagnosticMode;
+  }
+
+  /**
+   * Get diagnostic information
+   * @returns Diagnostic info object
+   */
+  getDiagnosticInfo(): { metrics: string; eventCount: number; diagnosticMode: boolean } {
+    return {
+      metrics: this.exportMetrics(),
+      eventCount: this.eventLogs.length,
+      diagnosticMode: this.diagnosticMode
+    };
+  }
+
   private trimMetrics(): void {
     if (this.performanceMetrics.fetchTimes.length > this.maxMetrics) {
       this.performanceMetrics.fetchTimes = this.performanceMetrics.fetchTimes.slice(-this.maxMetrics);