fix(array): honor struct offsets in constructors and validity masks

[fallintoplace]

Struct convenience construction could retain the full child length after applying a non-zero offset, causing child slicing to run past the end. Parent validity masking also addressed logical row positions instead of the child bitmap's physical offset.

This change validates struct offsets, uses length-offset for the constructed array, applies the existing null-bitmap validation to the names-based constructor, and masks child validity at physical bitmap positions. It also sizes replacement bitmaps for leading child offsets.

Tests:

• go test ./arrow/array -run 'Test(NewStructArrayWithNullsValidatesOffset|StructStringMasksParentValidityAtPhysicalOffset|StructArrayStringerMasksRequiredChildWithoutNullBitmap)'
• go test ./arrow/array