Drop the "Child Subject" term; record what a page's Subjects are

[JeroenDeDauw]

Fixes #959

A page holds Subjects, one of them optionally the Main Subject; sharing a
page is storage, not a relationship, so the other Subjects need no term of
their own. The glossary still called them Child Subjects, which implied a
link to the Main Subject that only an explicit Relation carries.

Drop the term from the glossary (with an Avoid entry), from the Draft
ADRs 28 and 31, and from planning/Relations.md; the decisions themselves
are unchanged. The planning doc also records where in-flow-created
Subjects land now that a Subject can be moved, carries the page-scoped vs
free-standing question from the issue so closing it does not lose the
question, and points at the identifier rename that follows
(#1367).

Considered, omitted: the identifier and API renames themselves and the
API docs that describe them, tracked in
#1367 for after
#1265 lands. ADR 7 keeps
its wording as an accepted record.

_{AI-authored — Claude Code, Fable 5 (max); from @JeroenDeDauw's #959 decision and an in-session brief to keep it minimal, one redirection; not yet human-reviewed; blind-judge pass applied, links checked, lines within 120.}