feat(custom): model reader-authored games and events

The data layer for PRD F13, with no UI and no behaviour change yet.

src/shared/custom.ts defines the two key spaces, their schemas, and the
projection into what the views read. Two properties are the point of it:

- A reader's event id is random, not derived from their title. They can type a
  scraped event's exact name and date, which under ${game}:${slug}:${date} is a
  byte-identical key — one completion mark and one streak silently shared by two
  events. Randomness also means renaming their own event never moves its id.
- A reader's event carries no sourceUrl, so a hand-entered date can never be
  attributed to a source, and claims no region split, because they entered one
  instant and inventing three would fabricate two of them.

The rest is widening what was GameId-shaped into a lane that may be one of
theirs: clockFor takes the boundary fields structurally so their events run on
the identical countdown rather than a second one, day keys fall back to the
regional default for a lane with no server map, and gameMeta becomes a context
resolver so metaFor stays pure and total — a lane can outlive its game when an
import carries an event whose game did not come with it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/client/components/GameFocus.tsx

@@ -1,5 +1,5 @@
-import { gameMeta } from "../../shared/games.ts";
-import type { GameId } from "../../shared/schema.ts";
+import { useGameMeta } from "../state/gameMeta.tsx";
+import type { LaneId } from "../../shared/custom.ts";
 
 /**
  * One game at a time.
@@ -24,16 +24,17 @@ export function GameFocus({
   onAdvance,
 }: {
   /** Games the reader has switched on, in feed order. */
-  games: GameId[];
-  focus: GameId | null;
+  games: LaneId[];
+  focus: LaneId | null;
   /** Outstanding rows per game, so a chip says whether it is worth a visit. */
-  counts: Partial<Record<GameId, number>>;
+  counts: Partial<Record<LaneId, number>>;
   total: number;
   /** Where "next" goes — null means back to all games. */
-  next: GameId | null;
-  onFocus: (game: GameId | null) => void;
+  next: LaneId | null;
+  onFocus: (game: LaneId | null) => void;
   onAdvance: () => void;
 }) {
+  const gameMeta = useGameMeta();
   // With one game there is nothing to focus down to, and the bar would just be
   // a chip that does nothing.
   if (games.length < 2) return null;