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

CHANGELOG.md

@@ -4,6 +4,10 @@ One line per change, grouped by the release that shipped it. Newest
 first. A release is a Cargo.toml bump plus a `v<version>` tag; the
 content repos pin the tag they run (see the end of this file).
 
+## Unreleased
+
+- Storylines (`src/content/variants.rs`, `docs/storylines.md`): 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 (the `portal_storyline` cookie, a year) and reads it on every page and in every mail about their case. What the machine reads - a field's `name`, `options`, states, groups, paths - never varies. The lint refuses a list of another length. Records and answer events carry `storyline`, and `GET /report` gives counts per bucket, per storyline, per state, and nothing else.
+
 ## 0.5.10 (2026-09-30)
 
 - The lint lets an invite form collect a required `phone` in place of a required `email`: memberships know a person by number first.