feat: add unified screenshot diff with layout-shift compensation

[jsfez]

Summary

Adds diffScreenshots, a single-pass screenshot comparator designed to replace odiff-bin in the Argos backend (companion PR in argos-ci/argos), plus computeRowAlignment exposing the row alignment on its own.

• Pixel diff in Rust: pixelmatch algorithm (YIQ color distance, alpha blending over white, antialiasing detection). Both Argos thresholds (base 0.15 / color-sensitive 0.0225) are evaluated in a single loop over in-memory images — the previous pipeline spawned odiff twice and decoded each PNG four times.
• Layout-shift compensation built in: row alignment via patience diff on quantized (5 bits/channel) row hashes; when a trustworthy vertical shift is found (min-run, coverage, segment-count and residual-ratio gates), the base image is realigned by an in-memory row map. Inserted sections are painted solid red on the mask, deletions get a 4px seam marker, and both are reported (insertedRows/deletedRows) so the caller can score them explicitly.
• Parallelized with rayon: image decode, diff rows and row hashing. Benchmark on Argos' big-images fixture: 66ms vs 200ms for the two parallel odiff spawns it replaces (×3).
• Masks are only encoded/written when they have content.

odiff parity

Validated against the full Argos fixture corpus (24 cases): the base-threshold pass is bit-identical to odiff on identical inputs; the color-sensitive pass diverges by +0.1–2% relative (OCaml vs Rust float precision at a very low threshold), with identical gate verdicts everywhere.

Tests

• 12 Rust unit tests (cargo test): alignment (insertion, deletion, repeated rows, min-run folding, width mismatch) and diff (color math, dual threshold, transparent padding, row-map realignment, end-to-end layout shift).
• 12 vitest tests including 6 new diffScreenshots specs on synthetic PNGs (masks content asserted pixel by pixel) and the pre-existing fingerprint spec, unchanged.

Release note

Version bumped to 0.3.0 — the Argos backend PR depends on this version being published to npm.

🤖 Generated with Claude Code