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]>

src/client/App.tsx

@@ -17,7 +17,7 @@ import { useAppUpdate } from "./state/useAppUpdate.ts";
 import { useMarkSet } from "./state/useMarkSet.ts";
 import { useProgress } from "./state/useProgress.ts";
 import { useDailyLog, type DailyLogMap } from "./state/useDailyLog.ts";
-import { usePrefs, type View } from "./state/usePrefs.ts";
+import { adoptNewLanes, usePrefs, type View } from "./state/usePrefs.ts";
 import { useCustom } from "./state/useCustom.ts";
 import { compareRows, SORT_MODES, type Activity, type SortMode } from "./state/sort.ts";
 import {
@@ -206,6 +206,26 @@ export function App() {
     [allRows, custom.lanes],
   );
 
+  /**
+   * A lane the reader has never been offered starts switched off.
+   *
+   * Adding a source is our decision, not theirs, and a reader who plays two
+   * games did not ask for the other twelve. So a lane that is new to *them*
+   * is recorded and hidden, and the games chips in settings are where they
+   * take it up — the one place that lists every lane, on or off.
+   *
+   * The seeding branch is the whole reason this is safe: an existing reader
+   * has no `knownGames` at all, and treating that as "has been offered
+   * nothing" would switch off every game they already read. Absent means
+   * unrecorded, so the first pass records what is already on their screen and
+   * changes nothing else.
+   */
+  useEffect(() => {
+    if (state.status !== "ready") return;
+    const patch = adoptNewLanes(games, prefs.knownGames, prefs.hiddenGames);
+    if (patch !== null) update(patch);
+  }, [state.status, games, prefs.knownGames, prefs.hiddenGames, update]);
+
   /** Games the reader plays, in feed order. The focus bar rotates through these. */
   const enabled = useMemo(
     () => games.filter((g) => !prefs.hiddenGames.includes(g)),
@@ -311,8 +331,8 @@ export function App() {
   }
 
   // First run: ask which games before showing a calendar full of ones they
-  // don't play. Stored as hiddenGames (the inverse) so a game added later shows
-  // up by default rather than staying invisible.
+  // don't play. Stored as hiddenGames (the inverse); what happens to a game
+  // added *later* is decided by `knownGames` above, not by this shape.
   if (!prefs.onboarded) {
     return (
       <GameMetaProvider value={gameMeta}>