Why (M2 follow-up; invariant 4, no trading): the previous commit let a moved+edited unit's
new-looking hits spend any departed occurrence of its origin unit by line text, so a swallow
dropped at the top of `legacy` paid for one added at the bottom of the moved copy (exit 0;
red on the previous commit), while the same relocation in place blocks. A unit matched
across files (pass 5) now compares its lines with its origin unit's (SequenceMatcher, code
only), exactly like an edit in place: hits on unchanged lines are old, the rest are new.
The departed-occurrence pool is now module-level only.
ae59e08534