Add playground examples

[GreenPioneer]

Summary

• refactor plugin registry to avoid initialization error and load built-in calculators separately
• expose a reusable fetchMetrics helper and include a test to ensure the playground example runs
• depend on bottleneck to satisfy rate limiter requirements
• allow playground fetch script to ignore self-signed certificates via --insecure and --base-url
• add tests demonstrating that disabling TLS verification bypasses self-signed cert errors
• ensure the project is built before running tests so playground examples execute correctly

Testing

• pnpm build
• pnpm test
• pnpm lint (fails: A require() style import is forbidden, 39 errors)

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https://chatgpt.com/codex/tasks/task_e_68960a636e308330aaa45a55cf2837c3