Add HTTP TRACE enabled check (#109)

[jthack]

Summary

• send TRACE probe with custom marker header and detect echo responses
• raise medium finding when server returns 200 and reflects the header
• expose new check id in registry and Balanced preset (disabled by default)

Testing

• pnpm lint
• pnpm typecheck
• pnpm test -- --match 'HTTP TRACE enabled'

Closes #109