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/state/usePrefs.ts

@@ -1,5 +1,6 @@
 import { useCallback, useEffect, useState } from "react";
-import type { GameId, Region } from "../../shared/schema.ts";
+import type { LaneId } from "../../shared/custom.ts";
+import type { Region } from "../../shared/schema.ts";
 import { guessRegion } from "../../shared/time.ts";
 import type { SortMode } from "./sort.ts";
 import { KEYS, readJson, writeJson } from "./storage.ts";
@@ -7,7 +8,7 @@ import { KEYS, readJson, writeJson } from "./storage.ts";
 export interface Prefs {
   region: Region;
   /** Games the reader has switched off. Stored as hidden so a newly added game shows up by default. */
-  hiddenGames: GameId[];
+  hiddenGames: LaneId[];
   /**
    * One game to look at right now, or null for all of them.
    *
@@ -16,7 +17,7 @@ export interface Prefs {
    * obeyed (`resolveFocus`), so it can never leave the reader on a blank page
    * with no visible cause.
    */
-  focusGame: GameId | null;
+  focusGame: LaneId | null;
   /** How the list is ordered. Deadline order is the default and the fallback. */
   sort: SortMode;
   /**
@@ -66,7 +67,7 @@ export function usePrefs() {
     setPrefs((prev) => ({ ...prev, ...patch }));
   }, []);
 
-  const toggleGame = useCallback((game: GameId) => {
+  const toggleGame = useCallback((game: LaneId) => {
     setPrefs((prev) => ({
       ...prev,
       hiddenGames: prev.hiddenGames.includes(game)