feat: add experimental monitor mode to BasicCrawler

[hiepau1231]

Fixes #2680

Continues the work from #2692, addressing all reviewer feedback and adding missing tests.

What changed

packages/core/src/crawlers/monitor.ts

• Guard ETA against going negative when finished > total (approximate counts from RequestManagerTandem) — Math.max(0, remaining)
• Show CPU: N/A on Windows where os.loadavg() always returns [0,0,0]
• Call .unref() on the interval so the monitor cannot prevent process exit

packages/basic-crawler/src/internals/basic-crawler.ts

• Add monitorOptions?: MonitorOptions to BasicCrawlerOptions so callers can configure intervalSecs and other options without monkey-patching
• Pass this.monitorOptions through to the Monitor constructor
• Skip the statusMessageLoggingInterval periodic logger when monitor: true is active to avoid plain log lines being interleaved with ANSI cursor-control sequences

test/core/crawlers/monitor.test.ts

• 4 new unit tests: negative-ETA guard, Windows CPU N/A label, intervalSecs config, improved cursor-overwrite assertion (15 total, all passing)

All 65 existing basic_crawler.test.ts tests continue to pass.

Usage

const crawler = new BasicCrawler({
  monitor: true,
  monitorOptions: { intervalSecs: 2 },
  async requestHandler({ request }) { /* ... */ },
});
await crawler.run(['https://example.com']);