feat(core): P3.1-A Add metrics contract infrastructure

Created src/core/metrics.ts with:
- PerformanceMetric interface
- MetricsCollector interface
- InMemoryMetricsCollector implementation

Updated src/core/index.ts to export metrics.

Note: Web stub (src/web.ts) doesn't need timing instrumentation since it throws immediately. Real instrumentation will be in platform implementations (P3.1-C).

Verification:
- TypeScript compiles ✅
- Core purity check passes ✅
- No new dependencies ✅

src/core/index.ts

@@ -61,3 +61,10 @@ export {
   isValidTimestamp,
 } from './guards';
 
+// Metrics
+export {
+  type PerformanceMetric,
+  type MetricsCollector,
+  InMemoryMetricsCollector,
+} from './metrics';
+

src/core/metrics.ts

@@ -0,0 +1,59 @@
+/**
+ * Core Metrics
+ * 
+ * Performance metrics contract and lightweight collector.
+ * Platform-agnostic, no dependencies.
+ * 
+ * @author Matthew Raymer
+ * @version 1.0.0
+ */
+
+/**
+ * Performance metric entry
+ */
+export interface PerformanceMetric {
+  /** Operation name (e.g., 'schedule.create', 'recovery.coldStart') */
+  operation: string;
+  /** Duration in milliseconds */
+  duration: number;
+  /** Timestamp when metric was recorded (milliseconds since epoch) */
+  timestamp: number;
+  /** Whether operation succeeded */
+  success: boolean;
+  /** Optional metadata */
+  metadata?: Record<string, unknown>;
+}
+
+/**
+ * Metrics collector interface
+ */
+export interface MetricsCollector {
+  record(metric: PerformanceMetric): void;
+  getMetrics(): PerformanceMetric[];
+  clear(): void;
+}
+
+/**
+ * Lightweight in-memory metrics collector
+ * No dependencies, platform-agnostic
+ */
+export class InMemoryMetricsCollector implements MetricsCollector {
+  private metrics: PerformanceMetric[] = [];
+  private maxMetrics = 100;
+
+  record(metric: PerformanceMetric): void {
+    this.metrics.push(metric);
+    if (this.metrics.length > this.maxMetrics) {
+      this.metrics = this.metrics.slice(-this.maxMetrics);
+    }
+  }
+
+  getMetrics(): PerformanceMetric[] {
+    return [...this.metrics];
+  }
+
+  clear(): void {
+    this.metrics = [];
+  }
+}
+

src/web.ts

@@ -332,14 +332,17 @@ export class DailyNotificationWeb implements DailyNotificationPlugin {
   }
 
   async createSchedule(_schedule: CreateScheduleInput): Promise<Schedule> {
+    // Web stub: no timing needed (throws immediately)
     this.throwNotSupported();
   }
 
   async updateSchedule(_id: string, _updates: unknown): Promise<Schedule> {
+    // Web stub: no timing needed (throws immediately)
     this.throwNotSupported();
   }
 
   async deleteSchedule(_id: string): Promise<void> {
+    // Web stub: no timing needed (throws immediately)
     this.throwNotSupported();
   }