docs(lessons): give the round-trip guard the measured growth, not a remembered one (#4272)

The module docstring said the defect grew the worst real body by 520
characters over ten edits. That number was recalled, not measured, and it is
wrong — as was the 2500 I then got by deriving it on paper instead of
running it.

Replayed lesson #4207's actual stored shape through the old code: 607
characters per edit, exactly the length of the trigger's paragraphs 2 and 3
with their separators, taking the body from 1291 to 7361 over ten edits.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

tests/test_lesson_insight_round_trip.py

@@ -22,8 +22,13 @@ composer writes all of it after the marker, so paragraphs 2..n were ordinary
 lines that the filter had no reason to touch. They came back as "the
 insight", the full trigger was prepended again, and the stored lesson gained
 (n-1) paragraphs per update — silently, compounding, and in the half of the
-document that does the retrieving. On the corpus's worst real shape it grew
+document that does the retrieving.
-the body by 520 characters over ten round trips.
+
+Measured on the one real row this damaged (lesson #4207, a three-paragraph
+trigger): the body grew by exactly 607 characters per edit — the length of
+paragraphs 2 and 3 with their separators — taking it from 1291 characters to
+7361 over ten edits, at which point the trigger appears eleven times and the
+insight is 17% of the record.
 
 WHY A MARKER COULD NOT HAVE FIXED IT. There is no end-of-trigger marker in
 the body and adding one would not repair a row already written without it.