feat(prefs): a game we add arrives switched off

Adding a source is our decision, not the reader's. Eleven lanes became
fourteen this week, and for someone who plays two that is not a feature
arriving — it is their calendar filling with events they will never open,
which is the thing the first-run picker exists to prevent.

`knownGames` records every lane a reader has been offered; a lane missing from
it is new to them, so it is recorded and hidden on sight. The games chips in
settings list every lane, on or off, which is where they take one up.

The case that had to be right is the reader who installed before any of this
existed: they have no `knownGames` at all, and reading that as "has been
offered nothing" would switch off every game they already read. Absent means
unrecorded — the first pass records what is already on their screen and
changes nothing else. Lanes they invented are recorded but never hidden;
typing a game in is asking for it.

The decision is a pure function so this is provable rather than watched for.
One real cost, stated in the PRD rather than hidden: a reader whose game
finally arrives is not told, which makes the colophon roadmap matter more.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

AGENTS.md

@@ -470,6 +470,13 @@ to an open page). Four things hold it up:
   on the timeline — so the *sorting groups, it never reorders* rule below holds for what is hidden
   exactly as it does for what is shown. Expanding is per-visit state, not a stored preference: it is
   something a reader does while reading one list, not a statement about how the app should work.
+- **A game we add arrives switched off, and `knownGames` absent means *unrecorded*.** Adding a
+  source is our decision; a reader who plays two games did not ask for the other twelve. So a lane
+  missing from `prefs.knownGames` is new *to them* and is hidden on sight (`adoptNewLanes`, PRD F8).
+  The trap is the other reading: every install from before this existed has no `knownGames` at all,
+  and treating that as "has been offered nothing" switches off every game they already read. Seeding
+  records what is on their screen and changes nothing else. Lanes they invented (`mygame:`) are
+  recorded but never hidden.
 - **Which view opens is the reader's answer.** `prefs.view` is asked once on the first run (PRD F8)
   and written by the tabs from then on. It was component state, which meant a reader who preferred
   the timeline was put back on the list by every reload, with nothing to blame but the app