Animate two carts of different mass bouncing elastically on an air track in one self-contained HTML file

# Animate two carts of different mass bouncing elastically on an air track in one self-contained HTML file Make a small animated scene: a air track with two carts. Write one file, `out/scene.html`, that works opened straight from disk with no network access. `spec.json` gives the page title, the stage size and the scene parameters. **Looks are not scored**: the page is opened in a headless browser (800x600 window, UTC time zone, network blocked) and checked by objective tests only. ## Scene contract - Draw the scene on one `<canvas>` or `<svg>` stage of `width` x `height` CSS pixels (spec.json), visible on load, with `<title>` containing spec.json `title`. - Animate continuously from load (`requestAnimationFrame` or timers); no click is needed unless stated below. - Expose `window.sceneState()`, returning the scene's current state in stage pixel coordinates (origin top-left of the stage): `{carts: [{name, x, v}, ...]} (x = left edge of the cart along the track, 0..track_px - cart_width_px; v in px/s, positive = right)`. Extra fields are fine. It must describe what is actually drawn. - At time 0 (`performance.now()` 0) cart A and cart B are at their spec.json `x` with velocity `v`. There is no friction. Cart-cart collisions are perfectly elastic with the given masses, and both ends of the track bounce a cart back at the same speed. `sceneState()` must return the state at the moment it is called. - The page must log no errors and throw no exceptions. Keep the file at most 200,000 bytes; no external scripts, fonts, images or URLs (inline everything). ## Checks (partial credit = share passed) 1. `self_contained`: `out/scene.html` exists, is at most 200,000 bytes, references no http(s) or protocol-relative URL, and requests nothing from the network when opened. 2. `has_canvas_or_svg`: a visible `<canvas>` or `<svg>` of at least 300x200 CSS pixels is on the page. 3. `frames_change`: over 3 s, some later screenshot differs from the first in at least 0.1% of pixels, and at least 3 of 30 screenshots differ. 4. `motion_between_collisions`: `carts` names exactly A and B; between samples where neither velocity changes each cart moves v × Δt within 1.5 px; carts stay on the track and never overlap (by more than 0.5 px). 5. `elastic_collisions`: when only one cart's velocity changes between samples it reverses at an end stop (within 1%); when both change, the new velocities match a perfectly elastic collision of `mass_kg` A and B within 2% of |vA| + |vB|; at least one cart-cart collision is seen in 3 s. 6. `no_console_errors`: no uncaught exception, `console.error` or browser error is logged during the run. ## How this run works - Work on your own: nobody will answer questions in this task, and writing to `ask/` counts against the run. - Write only under `out/`. Create `out/` if it is missing. Inputs are synthetic; don't fetch anything from the network. - Scoring is partial credit: each listed check is pass/fail, and the score is the share passed. Numbers are compared to within 0.005 unless the task says otherwise.