[Analysis] AlterTableDecorator.php: $insertStart[$j] loop-bound findings stem from mago flow-analysis limitation #69

Context

Part of #60.

Data

mago analyze flags possibly-undefined-int-array-index, possibly-null-argument, and possibly-null-operand around $insertStart[$j] in processAddColumns()/processChangeColumns(). $insertStart is returned by getSqlInsertOffsets() with a guaranteed array{0: int, 1: int, 2: int, 3: int} shape, and $j only ever takes values 0-3 (set via the switch in the outer loop, then incremented by for (; $j < $insertStartCount; ++$j)). mago cannot correlate the for loop's incrementing bound against the array's guaranteed size, the same class of limitation already documented for Statement.php (#61) and Result.php's dereference loop (#66).

These findings will be captured in a mago baseline rather than fixed or suppressed inline.