feat(perf): implement comprehensive SDK performance and load testing suite (closes #982)

[Ranjeet2063]

Summary

Closes #982

Requirements & Implementation

• Core Performance Engine (src/perf/loadTester.ts): Implements LoadTester supporting concurrent asynchronous worker pools, progressive ramp-up rate limiting, operation timeouts, and high-precision latency measurement (performance.now()).
• Comprehensive Percentiles & Metrics: Calculates throughput (TPS), error rates, error category classification, and complete percentile distribution (min, max, mean, p50, p90, p95, p99).
• Test Suite (test/loadTesting.test.ts):
  • Validates concurrency bounds and peak in-flight worker limits.
  • Verifies fault isolation, failure aggregation, and error rate calculations.
  • Asserts strict timeout enforcement and budget bounds.
• Exports (src/perf/index.ts): Exposes typed LoadTester, LoadTestConfig, and LoadTestResult interfaces.

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