The coverage ratchet reads `pct >= 99.99`, and with the last carried bodies
typed it cannot usefully be raised: the only opaque items left are the MT19937
block's two in every save, so the number is 2/items and it still ranges from
99.99400 on a 532 KB save to 99.99746 on a 1.17 MB one. Moving the bar to
99.995 would fail a save that types perfectly and pass a save twice its size
that had picked up a whole new opaque body. A percentage ratchet on a
fixed-size debt is a ratchet on save size.
So the percentage stays where it is and a stronger line goes beside it: the
opaque set must be exactly the two RNG items. That breaks on the save that has
the new body, and `opaque_by_tag` names it.
test_domains: 43 saves give 755 fields observed, 538 varying, 217 constant,
against a baseline of 490 measured at 22 saves. Nineteen of the new fields are
the bodies typed in the previous commit and only two of them (FTPShID,
FTPDesID) have ever been seen to move -- which is why the constants grew faster
than the varying fields, and is worth leaving visible.
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