Fix row alignment when column selection heuristic excludes distinguishing columns

[jamesscottbrown]

When a table has more than 5 columns, daff falls back on heuristics. These can fail, causing daff to incorrectly report that a row was deleted and another inserted, rather than reporting that a row was modified. This is Issue #216.

The PR is a proposed fix, created by an LLM (claude-opus-4.5). It looks plausible to me and the tests pass, but I am unfamiliar with the Haxe language and table-comparison logic is somewhat complex.

It adds a fallback matching mechanism in coopy/CompareTable.hx that:

1. Identifies unmatched rows that are "sandwiched" between matched rows at the same relative positions
2. Verifies the candidate row in B has no duplicates (to avoid ambiguous matching)
3. Checks that more than half the columns match between the rows
4. If all conditions are met, links the rows as a match