feat(family): an idea links to the rule topic holding its norms - set_family_topic, the readouts name the rules, the idea row shows the link (milestone 463 step 1 gap, found in step 7, #4993)

Step 1 added family_ideas.topic_id but nothing could set it, so the note<->topic
link the milestone promised existed only as a column. set_family_topic links or
unlinks (0) a topic in a rulebook the caller owns, one idea per topic; the
version does not move. get_family_idea and get_family_adoption list the topic's
rules; list_family_ideas and the Family page name the topic. The model
docstrings no longer claim the topic's rules are platform-scoped in retrieval.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scribe/models/family.py

@@ -23,8 +23,9 @@ The tables, and the one job each does:
 - `family_ideas` — a note's family state: candidate, canon or retired, its
   applicability test, its canon version, its linked rule topic.
 - `family_idea_platforms` — the platforms an idea is for. The ONLY scope
-  source: a linked topic takes its scope from here rather than carrying its
-  own, so the two can never disagree.
+  source: a linked topic carries no platforms of its own, so the two can
+  never disagree. The scope decides which projects owe the idea an answer;
+  the topic's rules still reach a session by ordinary rule retrieval.
 - `family_idea_references` — the reference implementations.
 - `family_adoptions` — one row per (project, idea): the project's answer.
 - `family_decisions` — the append-only log of every promotion and every
@@ -165,8 +166,9 @@ class FamilyIdea(Base, TimestampMixin):
     comparing the two numbers, never stored as a flag that could go stale.
 
     `topic_id` is the rule topic holding the idea's binding norms, if it has
-    any (the note↔topic link #3236 asked for). The topic takes its scope from
-    this idea; it never carries platforms of its own.
+    any (the note↔topic link #3236 asked for), set by `set_topic`. The topic
+    never carries platforms of its own; the idea's readouts list its rules,
+    so an assessment judges a project against them.
     """
 
     __tablename__ = "family_ideas"