docs: Migrationsdokument haelt das Vorher fest und verweist auf die .template-Form, Rest-Abschnitt aus types/source.md entfernt (#107)

Files changed:
- CHANGES.md
- VERSION
- instructions/migrate-corpus.md
- instructions/migrations/6.0.0-type-guidance-split.md
- types/source.md

instructions/migrate-corpus.md

@@ -120,6 +120,16 @@ Write it for a reader who has the new machinery and the old content, and who is
 changed, which pages are affected, how to tell a migrated page from an unmigrated one, and what
 `migrate verify` should report when it is done.
 
+**A verification step names its own baseline, and does it in an earlier step.** Where the
+document asks that something "read the same as before" - a composed `types describe` answer, a
+rendered index, any command's output - it says what to capture, where to put it, and at which
+point, so the check is a `diff` rather than a memory. Step 4's `migrate verify` needs none of
+that: its baseline is the last commit, which git holds whether or not anyone thought to keep it.
+A migration that changes machinery rather than `kb/` pages has no such baseline, and that is
+exactly where the unfalsifiable version has already slipped through - the 6.0.0 type-guidance
+split asked for output that "must read the same", named nothing to compare it against, and a
+stray section in the middle of one type-spec survived a check made in good faith.
+
 **Baseline: 1.0.0.** Migrations that predate it - the type-system move, the `confidence_base`
 backfill, the German section headings, the translation itself - have no documents and will not
 get any. An instance older than that is re-exported, not migrated.