[benjamin-small]

Slice 2 of the JPEG compression lab: the engine lab and stages 1–4 at #/jpeg. The lab isn't linked from the nav or home page yet; that comes in slice 4.

• Spec: docs/superpowers/specs/2026-10-15-jpeg-lab-design.md (see the Plan 2 amendment)
• Plan: docs/superpowers/plans/2026-10-16-jpeg-lab-2-stages.md

What's in it

Engine

• Three generic hooks: update_rule_config_with_image (pixels as a typed array), rule_inspect, rule_bytes. They don't change any other lab.

jpeg rule

• One encode per config; the engine clock walks the scan MCU by MCU.
• select_block is the only action.
• The first selection is the block with the most detail.

Visualization

• A TextureQuad renderer, and JpegViz, which draws whatever panels the page lays out (with optional zoom crops).

Page

• A nine-step pipeline rail.
• Image controls: built-ins, upload, quality, chroma and Huffman; settings changes re-encode and restore the scan position.
• Readout and Save .jpg.
• Stages 1–4 with live formulas:
  • Color: a rotating colour cube from RGB to YCbCr axes.
  • Subsample: zoomed chroma, as measured next to as kept.
  • Blocks: the block grid and the selected block's numbers.
  • DCT: basis atlas, coefficients, a k-term rebuild, energy curve and "why cosines?".
• Share links: img, q, s, h, stage, b.
• Keyboard: the arrow keys move between stages, and move the block when the image is focused.

Tests

• cargo test --all-features: jpeg 85, viz-core 293.
• wasm-pack test --chrome --headless: 46.
• Vitest: 478.
• fmt, clippy (no new warnings) and svelte-check are clean.
• Checked in the browser at 1440 and 375 px, light and dark.