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) <[email protected]>

src/client/state/gameMeta.tsx

@@ -0,0 +1,36 @@
+import { createContext, useContext } from "react";
+import type { CustomGames, LaneId } from "../../shared/custom.ts";
+import { metaFor, type GameMeta } from "../../shared/games.ts";
+
+/**
+ * How a component turns a lane id into a name, a short label and a hue.
+ *
+ * It used to be a direct import of `gameMeta`, which could only answer for the
+ * games in `GAMES`. Now a lane can also be one the reader invented (PRD F13),
+ * and those live in their browser rather than in a module — so the resolver has
+ * to come from somewhere with access to that state.
+ *
+ * A context rather than module-level mutable state: `metaFor` stays pure and
+ * takes the reader's games as an argument, and nothing renders off a registry
+ * that some other part of the app has been quietly writing to.
+ */
+export type MetaResolver = (id: LaneId) => GameMeta;
+
+const NO_CUSTOM_GAMES: CustomGames = {};
+
+const GameMetaContext = createContext<MetaResolver>((id) =>
+  metaFor(id, NO_CUSTOM_GAMES),
+);
+
+export const GameMetaProvider = GameMetaContext.Provider;
+
+/**
+ * The lane resolver for this tree.
+ *
+ * The default answers for tracked games only, which is the correct answer
+ * anywhere the reader's own games cannot appear — and a safe one everywhere
+ * else, since `metaFor` is total.
+ */
+export function useGameMeta(): MetaResolver {
+  return useContext(GameMetaContext);
+}