Calculate the acid-test ratio for Company YZ, given its current assets of $1 million, excluding inventories, and current liabilities of $500,000. Describe the acid-test ratio as a liquidity ratio used to measure a company's ability to meet short-term obligations with its most liquid assets. Discuss the significance of a high or low acid-test ratio in assessing liquidity, financial health, and its implications for short-term solvency and operational stability.