Storylines: the same site told several ways, one per visitor, counted

Any text a person reads may be a list, one entry per storyline, every
list the same length. A visitor is given one storyline for the whole
site and reads it on every page and in every mail about their case.
Records and events carry the storyline; GET /report gives counts per
bucket, storyline and state, and nothing else.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GT37Z1Xtfd9pUuQtMTg6Yt

docs/SUMMARY.md

@@ -8,6 +8,7 @@
 - [Content reference](content-reference.md)
 - [State machines](state-machines.md)
 - [Access](access.md)
+- [Storylines](storylines.md)
 - [Memberships](memberships.md)
 - [Operations](operations.md)