feat: add Rust conformance test infrastructure

[lilith]

Summary

• Add self-contained conformance tests comparing jxl-rs output against the official libjxl conformance test suite
• Add GitHub Actions workflow that reports results without blocking PRs
• Tests auto-fetch conformance data from libjxl/conformance repo

Details

The conformance tests:

• Parse NPY reference files for pixel comparison
• Support all pixel formats (u8, u16, f16, f32)
• Cache conformance data in CI for faster runs
• Use #[ignore] attribute - run with cargo test conformance -- --ignored --nocapture

Currently passing 18/23 tests. Failures are due to missing features (CMS transforms, specific edge cases) that can be addressed in follow-up PRs.

Test plan

• cargo test passes (conformance tests are ignored by default)
• cargo test conformance -- --ignored runs conformance suite
• CI workflow runs and reports results in step summary

[github-actions]

Benchmark @ fbdf262

MULTI-FILE BENCHMARK RESULTS (4 files)
  CPU architecture: x86_64
  WARNING: System appears noisy: high system load (2.16). Results may be unreliable.
Statistics:
  Confidence:               99.0%
  Max relative error:        3.0%

Comparing: 63b76047 (Base) vs 840f3a14 (PR)

File	Base (MP/s)	PR (MP/s)	Δ%
bike.jxl	35.264	35.161	-0.29% ±2.0%
green_queen_modular_e3.jxl	9.070	9.049	-0.24% ±1.9%
green_queen_vardct_e3.jxl	29.494	29.361	-0.45% ±1.9%
sunset_logo.jxl	2.873	2.885	+0.40% ±0.9%

[veluca93]

What's the advantage of replicating the conformance workflow over using https://github.com/libjxl/conformance ?

[lilith]

That is what it fetches though? Not sure what you mean specifically. That repo is what has driven my patches and should be part of CI testing as a goal, not a breaking workflow for now.

…

On Mon, Jan 26, 2026, 12:33 PM Luca Versari ***@***.***> wrote: *veluca93* left a comment (libjxl/jxl-rs#660) <#660 (comment)> What's the advantage of replicating the conformance workflow over using https://github.com/libjxl/conformance ? — Reply to this email directly, view it on GitHub <#660 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAA2LHZIVWFNO5WNE2A6VFD4IZTZ5AVCNFSM6AAAAACS6CMXVCVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZTQMBRGM3DGNZSGI> . You are receiving this because you authored the thread.Message ID: ***@***.***>

[veluca93]

There is already a workflow for conformance tests though: https://github.com/libjxl/jxl-rs/blob/main/.github/workflows/pull_request.yml#L159

[zond]

That is what it fetches though? Not sure what you mean specifically. That repo is what has driven my patches and should be part of CI testing as a goal, not a breaking workflow for now.
…
On Mon, Jan 26, 2026, 12:33 PM Luca Versari @.> wrote: veluca93 left a comment (libjxl/jxl-rs#660) <#660 (comment)> What's the advantage of replicating the conformance workflow over using https://github.com/libjxl/conformance ? — Reply to this email directly, view it on GitHub <#660 (comment)>, or unsubscribe https://github.com/notifications/unsubscribe-auth/AAA2LHZIVWFNO5WNE2A6VFD4IZTZ5AVCNFSM6AAAAACS6CMXVCVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZTQMBRGM3DGNZSGI . You are receiving this because you authored the thread.Message ID: @.>

Note that we have the conformance tests as a breaking part of the workflow right now (the numpy + ICC parts are breaking, not the PNG parts). See https://github.com/libjxl/jxl-rs/blob/main/.github/workflows/pull_request.yml#L159