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/NextUp.tsx

@@ -1,4 +1,4 @@
-import { gameMeta } from "../../shared/games.ts";
+import { useGameMeta } from "../state/gameMeta.tsx";
 import { formatRemaining } from "../../shared/time.ts";
 import type { RowEvent } from "./EventRow.tsx";
 import { Meter, URGENCY_COLOR } from "./Meter.tsx";
@@ -26,6 +26,7 @@ export function NextUp({
   focused: string | null;
   onOpen: (id: string) => void;
 }) {
+  const gameMeta = useGameMeta();
   if (row === null) {
     return (
       <section className="border-b border-hairline px-4 py-8">