Cost Basis Calculator

Sorts trades by date first — a sell can only consume lots that existed before it, so order is what makes the calculation correct regardless of how rows were pasted. Matches each disposal against specific lots, tracks holding period per disposal so long-term and short-term gains are separated, and reports what remains held with its cost. When more is sold than was ever bought it says so rather than assuming zero cost, which would overstate the gain. Runs entirely in the browser — nothing is uploaded.