Starshell Circuit

Build a tiny browser game called "Starshell Circuit".

Game spec
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A Mario Kart style pseudo-3D kart race rendered from scratch on one 2D canvas: a segmented road with real perspective, elevation, banked curvature, rumble strips and depth haze, with shaded low-poly karts drawn back-to-front. Three laps against three (easy), five (medium) or seven (hard) CPU rivals that follow a racing line, brake for corners by proximity-weighted curvature, dodge peels and rubber-band toward the player. Drive with arrows/WASD; hold SHIFT/K through a corner to drift and release for a charge-tiered boost (blue, orange, red), hitting blue chevron pads for a free launch. ? boxes on the line give a homing green shell, a triple shell, a banana, lightning that flattens the field, a mushroom or star invincibility, weighted so last place pulls the better items. Off-track grass scrubs speed, centrifugal force pushes you wide, karts bump and scrub rather than crash. HUD: lap, live position, race timer plus best lap, an analogue speedometer, item slot and a minimap of every kart; finishing shows a full classification with per-rival time gaps. Sunset-violet look: gradient dusk sky, twin suns, parallax ridges and twinkling stars, palm/monolith/crowd-stand scenery, gantry arches over the track, spark and exhaust particles, screen shake, and synthesised engine tone plus countdown/hit/boost cues. Score is reported to the shell from lap crossings, drift boosts, item hits and final placement, scaled by difficulty.

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.