refactor: port lib layer to src/lib

classic-mode.js moves byte-for-byte. champion-data.js swaps the Next.js
env-var fetch prefix for asset() from $app/paths, which resolves to
/champions.json in dev and /loldle/champions.json in the deployed build.
getChampionImageUrl keeps returning an absolute Data Dragon URL, unwrapped.

Drops three exports with no callers: getAllChampions, getChampionByName,
getGuessedNames.

The characterization suite passes unchanged against the new location -- only
import specifiers moved, no assertions touched.

web/test/game-engine.test.js

@@ -7,7 +7,7 @@ import {
   loadUnlimitedStats,
   saveUnlimitedStats,
   submitGuess,
-} from "../lib/game-engine";
+} from "$lib/game-engine";
 import { AATROX, AHRI } from "./fixtures/champions";
 
 /** Minimal stand-in for compareChampions — the engine only stores what this returns. */