ci: say what the offender listing is, and how to find the new one

[fujibee]

CI only. One file, .github/scripts/check-enforced-assertions.sh. No product change, and no change to what passes or fails.

Why

Four people read this checker's output on one red tonight, and each drew a different wrong conclusion:

• the first file in the listing taken as the cause;
• a hundred-line difference between two heads read as a hundred new offenders;
• from that, a conclusion that the listing must be truncated;
• and one reader who had the answer in a file list produced for another purpose and did not recognise it.

Everyone eventually arrived by the same route — run the checker at each commit and watch where the count changes — and nothing in the output suggests it.

What was measured

The listing is not truncated. With the baseline forced low:

reported   638 unenforceable assertions
printed    638 lines

It is complete, unordered by novelty, and unlabelled. On a +1 that is 639 lines with nothing marking the one that matters. The hundred-line diff was line numbers shifting below an insertion, not new entries.

The change

check-enforced-assertions: 639 unenforceable assertions, baseline is 637 (+2).

2 assertions pushed this over. The list below is EVERY unenforceable
assertion in the tree, not the new one(s) -- nothing here marks which.
To find them: run this checker at the previous commit and compare the
counts, bisecting until the count changes. Comparing the listings does
not work; a line added anywhere shifts every line number below it.

Deliberately no new state

Storing the previous offender set beside the count would let this name the new entries outright. It also introduces a second artifact to keep in sync with the first, and a listing that disagrees with its own count is the same confusion wearing different clothes — on a checker four people just misread. The count stays the only authority; what changes is that the output stops implying otherwise.

If the stored-set version is wanted later, #892 has the argument for it.

Verified

At +1, +2, +9, at the baseline, and below it. The pass and below-baseline paths are untouched and still read as before.