[ntuple] Union merging can corrupt a late class field when its dictionary differs from the file #23683

In Union mode, a class field first appearing in a later source is reconstructed from the currently loaded dictionary. The merger then pairs source and destination child fields by position. A differing dictionary can therefore cause a successful merge to copy member pages to the wrong destination members.

Reproduction shape:

1. In a process declaring struct Rec { int fA = 11; int fC = 33; };, write one RNTuple with only an integer e and another with e and a Rec field called record.
2. In a separate process, declare struct Rec { int fA = 11; int fB = 22; int fC = 33; };.
3. Union-merge the file containing only e first, followed by the file containing record.

Expected: preserve the on-disk member layout or reject the incompatible schema cleanly.

Observed in a release build: the merge reports success, record.fB receives the source fC data and record.fC reads zero. Reordering members can also swap values; removal can throw an exception out of Merge. This predates #23646, which fixes a different late-field truncation/quantization problem.

A focused fix preflights newly reconstructed fields before adding them to the destination model and uses the existing Abort/Skip policy for mismatches. Schema conversion is a separate design question. Investigation and the proposed implementation used Codex and an independent native Claude Code review.