F2: cells beyond the header width are silently dropped in both loaders #82

Found in ART Milestone 0 (#53). Code read at main 032b522; not reproduced.

Problem

Both loaders iterate only over the header's columns:

• dynamic_loader.py:263-264: cell = raw[c] if c < len(raw) else ""
• te_loader.py:221: the same pattern.

A data row with more cells than the header loses the extra values, and they appear in no count or error. This contradicts the data-preservation constraint ("never silently discard columns") and the intent of BR-10 (reject rows that don't match the header).

Fix

Treat a row wider than the header as a row error ("row has N cells, header has M"), the same way a cast failure is reported. Add tests for both modes.

Part of ART M1. BR-34.

Generated by Claude Code