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/components/Welcome.tsx

@@ -1,6 +1,6 @@
 import { useState } from "react";
-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";
 
 /**
  * First run: pick your games.
@@ -15,12 +15,13 @@ export function Welcome({
   available,
   onConfirm,
 }: {
-  available: GameId[];
-  onConfirm: (chosen: GameId[]) => void;
+  available: LaneId[];
+  onConfirm: (chosen: LaneId[]) => void;
 }) {
-  const [chosen, setChosen] = useState<GameId[]>([]);
+  const gameMeta = useGameMeta();
+  const [chosen, setChosen] = useState<LaneId[]>([]);
 
-  const toggle = (id: GameId) =>
+  const toggle = (id: LaneId) =>
     setChosen((prev) =>
       prev.includes(id) ? prev.filter((g) => g !== id) : [...prev, id],
     );