Header alignment overrides unexpectedly change HTML body cell alignment #476

On unchanged upstream 268615a, tabulate([["item", 42]], headers=["Label", "Value"], tablefmt="html", headersalign=("center", "left")) changes the td styles to center/left as well as changing the th styles. Without the header-only override, the data cells correctly use left/right. headersalign is documented as a header-specific override, so numeric data should remain right-aligned and explicit colalign should remain honored. The fallback header alignment aliases the data alignment list and then mutates it. Six regression cases fail before the change; ten controls with independent global header alignment or already matching data styles pass. A copy of that list keeps the existing formatting but separates the two states. Investigated with AI assistance; no personal account-owner review or attestation is asserted.