Add reusable performance test suite for adapter benchmarking

[Copilot]

Adds an exported performanceTest() function so adapter authors can benchmark custom adapters against a standard set of operations. Includes a test running the suite against the Express adapter.

Changes

• src/plugins/server/performance-test.ts — New module with performanceTest() that benchmarks 6 operations: bulk write, query, get-by-id, bulk delete, sequential single-doc writes, and replication pull. Returns structured PerformanceTestResult[] with timing and ops/sec.
• src/plugins/server/index.ts — Re-exports performanceTest, PerformanceTestResult, PerformanceTestOptions
• test/unit/performance.test.ts — Runs the suite against RxServerAdapterExpress, logs results, asserts sanity
• test/unit.test.ts — Registers the new test file

Usage for custom adapters

import { performanceTest } from 'rxdb-server/plugins/server';

const results = await performanceTest({
    adapter: myCustomAdapter,
    createDatabase: () => createRxDatabase({ name: 'bench-' + randomToken(6), storage }),
    getPort: () => getAvailablePort(),
    batchSize: 50
});

for (const r of results) {
    console.log(`${r.name}: ${r.timeMs.toFixed(1)}ms (${r.opsPerSecond} ops/sec)`);
}

The PerformanceTestOptions interface takes an adapter, a database factory, a port provider, and an optional batchSize (default 30). Each test spins up its own server/collection and tears down after measurement.