Factorized tables zero-fill a 256 KB block up front, 512 KB cleared per constant query #1299

This work was produced with the help of language models.

Every FactorizedTable zero-fills a full 256 KB page for its first block, even when it holds one row. RETURN 1 creates two of them, the dummy scan's table and the result collector's, so each query clears 512 KB it never reads: about 13% of a warm RETURN 1.

Expected: a one-row table touches one tuple's worth of memory. Actual: every query pays for two 256 KB memsets.

Part of #1290. Fix in #1300.