One file is enough for a first ship.

Engines exist. You do not need one to prove the loop. GameDever’s preview is a single index.html on purpose — itch.io and Cloudflare Pages both eat that file.

What belongs in the file

Markup — a canvas or a few DOM nodes. A score. A restart control.
CSS — inline. Dark playfield. Big hit targets on a phone.
JS — input, update, draw. requestAnimationFrame or a small interval. Collision. Game over.
Touch — if it only has arrow keys, half the testers bounce off.
No CDN engines — a preview iframe and a locked-down host both fight extra scripts.

What can wait

Audio, particle editors, accounts, leaderboards on a server, ads. GameHub adds coins later; the first Snake clone does not need them. See the arcade walkthrough.

Generate a preview

If the iframe is blank, the file never finished — hit Generate again or paste from another AI.