Change Ring

Build a tiny browser game called "Change Ring".

Game spec
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A change-ringing puzzle on a tower of six bells. The row at the top of the screen is the sequence the bells rang last; each round you choose which adjacent pairs to swap, then the tower rings the new row and it scrolls down the ledger. The rule that makes it a puzzle: from one round to the next a bell may only move one place, so swaps must be non-adjacent, and every row must be different — ring a row twice and the sexton throws you out. Click a boundary between two bells to swap them, or press the number keys, or use the arrows plus Enter. Ringing the plain hunt (each bell dodging one place at a time) scores a per-round bonus, ringing a truth (no repeats) accumulates it, and completing a named peal of 24 rounds lights the tower. Score is rounds times the current streak plus a completion bonus. The HUD shows the target row, the number of distinct rounds rung and the active difficulty. Candlelit belfry look: timber frame, six bronze bells with clappers that swing as they sound, a stone ledger of rows in warm cream, and WebAudio struck-bell tones pitched per bell.

Hard constraints — the game will be embedded in a sandboxed iframe on a static gallery site, and a headless browser will screenshot it:

1. Output exactly ONE self-contained HTML file. All CSS and JS inline. Zero external requests: no CDNs, no external fonts, images, or sounds, no fetch/XHR/WebSocket. Any sound must be synthesized with WebAudio.
2. The iframe sandbox is "allow-scripts allow-pointer-lock" (plus an autoplay permission). So: no localStorage, cookies, or indexedDB (opaque origin — access throws); no popups, window.open, or navigation; no access to the parent page; alert()/prompt()/confirm() are silently blocked.
3. The playfield must be visible within 800 ms of load — that is when the screenshot is taken. No dead white screen. If there is a start screen, make it visually rich.
4. Controls: keyboard AND mouse must both work, plus on-screen touch buttons for phones. Call preventDefault() on game keys (arrows, space) so the page never scrolls.
5. The view is 16:9 and must scale to fill the viewport with no overflow or scrollbars.
6. Show a visible score. If the genre has a losing state, show a game-over screen with a restart (button and a key). Idle/clicker games skip game over but still need clear numeric feedback.
7. If you track a score, report it to the arcade shell so the player's best is remembered between visits: on every score change, call parent.postMessage({ source: "ai-arcade", type: "score", score: <number> }, "*"). It is safe to call it no matter what the parent does with it.
8. Keep it tiny: aim for under 400 lines, 60 fps, no frameworks. That 400-line target does not apply to 3D games — true 3D (a camera and perspective-projected world) or pseudo-3D (projected road/track, depth-sorted sprites) — which need their own software renderer and typically land at 800–1400 lines. The rest of this constraint still binds every game: 60 fps, no frameworks, no external assets, and no padding to reach a length.
9. Difficulty: the arcade shell passes the player's chosen difficulty as a URL query parameter. Read it with new URLSearchParams(location.search).get("difficulty") — the value is "easy", "medium", or "hard" (missing or unknown means "medium"). Make the three levels meaningfully different — speed, lives, enemy pressure, reaction time, whatever fits the genre; clicker/idle games with no losing state use starting resources and upgrade cost curves instead. Show the active level in the HUD. Without the parameter the game must be fully playable at its default (medium) tuning.

Return only the complete HTML file, no commentary.