Whole-file failure on validation error (strict mode)

If any row in a file fails schema validation, the entire file fails — no records are committed. The alternative (lenient mode: commit valid rows, discard bad ones) was rejected because partial commits make it impossible to reason about destination completeness. A table that received 9,800 of 10,000 rows looks identical to one that received all 10,000; downstream consumers cannot detect the difference without external knowledge of the source file. Strict mode means a FAILED file is an unambiguous signal: nothing landed, retry is safe, and the source data needs fixing. A dead-letter quarantine for bad rows is a future addition, not a default — adding it later does not require changing the strict-mode guarantee for well-formed files.