fix: resolve 50-100x slower add() on cloud storage (GCS/S3/R2/Azure)

Root cause: Storage type detection at setupIndex() relied on
this.config.storage.type which was never set after createStorage()
auto-detected the storage type. This caused cloud storage to use
'immediate' persistence mode instead of 'deferred', resulting in
20-30 GCS writes per add() operation (7-12 seconds instead of 50-200ms).

Fix: Added getStorageType() helper that detects storage type from
the storage instance class name (e.g., GcsStorage → 'gcs'), used as
fallback when config.storage.type is not explicitly set.

Also added:
- Performance regression tests (10 new tests)
- test:perf npm script for running performance tests

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

package.json

@@ -83,6 +83,7 @@
     "test:watch": "NODE_OPTIONS='--max-old-space-size=8192' vitest --config tests/configs/vitest.unit.config.ts",
     "test:coverage": "NODE_OPTIONS='--max-old-space-size=8192' vitest run --config tests/configs/vitest.unit.config.ts --coverage",
     "test:unit": "NODE_OPTIONS='--max-old-space-size=8192' vitest run --config tests/configs/vitest.unit.config.ts",
+    "test:perf": "vitest run tests/unit/performance --reporter=basic",
     "test:integration": "NODE_OPTIONS='--max-old-space-size=32768' vitest run --config tests/configs/vitest.integration.config.ts",
     "test:s3": "vitest run tests/integration/s3-storage.test.ts",
     "test:distributed": "vitest run tests/integration/distributed.test.ts",