Fetch wire boundaries and field-read errors

The Fetch/FetchWhere protocol represents each cell with a u16 byte length. Main already rejects values over 65,535 bytes and applies a 16 MiB response budget (introduced by 58a357a2). End-to-end tests now cover 65,535, 65,536 and 70,000-byte memo values in base-table Fetch/FetchWhere and SQL-cursor Fetch. The maximum representable value is returned intact; oversized values fail.

The new correction rejects field-read errors and unknown columns instead of returning a successful empty cell. SQL-cursor column names which would be truncated by the 64-byte lookup buffer fail rather than looking up a different name. Existing size guards are retained. No wire format or successful cell encoding changes. Broader SQL execution and filesystem race review remain open.