Signal Box

Build a tiny browser game called "Signal Box".

Game spec
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A railway signal box at a junction. Trains roll in from three coloured approaches and each is bound for a specific exit, printed on its side; you set the points by clicking a lever (or pressing 1-4, or arrows plus Enter) and clear the signals. A train commits to its route the moment it passes the outer home signal, so the decision has to be made before it gets there, and points cannot be thrown under a moving train. Routes conflict: two approaches that share a stretch of track cannot be cleared at once, and the interlocking refuses the lever with a clunk unless the section is free. Sent into the wrong exit, a train is lost; two lost trains ends the shift. A track diagram glows where a section is occupied, levers are colour-coded by the route they set, and a spare lever pulls the emergency stop that halts a train short of a conflict at the cost of punctuality. Score comes from trains delivered on time plus a conflict-free bonus, and each wave adds trains, shortens headways and closes one lever. Score is reported to the shell on every change. British signal-box look: a brown lever frame with brass handles, a track diagram of amber lamps on black, herringbone-patterned lever labels, and a clattering mechanical frame sound in WebAudio.

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.