BLD/DEV/CI: add and use Pixi package definitions

[lucascolley]

towards gh-24

The rough idea:

• the various build tasks in our workspace become proper Pixi package definitions which produce an xsf package which can be depended on in a Pixi workspace
• for variants of xsf for which we build the tests, we now install the tests in the package under share/xsf/tests/ rather than leaving them in the build directory
• the top-level workspace definition then just needs to hold non-building tasks, and environment definitions which use these variants of xsf

Next step:

• enable proper warm caching — currently waiting for fix: avoid rebuilds for mtime-only source changes prefix-dev/pixi#6703 to be released

[lucascolley]

moved to the file in the repo root

[lucascolley]

The cache miss is due to mtime:

  DEBUG source-build{package=xsf host_platform=linux-aarch64}: pixi_command_dispatcher::keys::source_build: artifact cache miss, rebuilding package=xsf key=jPjUa4XPohY reason=input file modified: /home/runner/work/xsf/xsf/CMakeLists.txt (mtime at build time 2026-08-10 16:08:45.721528754 UTC, now 2026-08-10 16:45:22.533354230 UTC)

[lucascolley]

@scipy/xsf-team I would greatly appreciate your input! The diff is a bit intimidating at the moment, but I'm mainly interested for now in whether this causes any problems for your current developer workflows. If you could check out this branch and try making some changes and running some tests, that would be great.

The main change is that there are no configure or build tasks any more — that is all handled automatically by Pixi. The intention is for rebuilds to happen only when needed and with good caching.

The workspace is significantly simplified, with just the following tasks:

❯ pixi task list
Task              Description
coverage          Generate HTML coverage report.
format            Format tracked C++ source and header files in place.
format-dry-error  Check C++ formatting and fail if any file needs changes.
open-coverage     Open HTML coverage report.
test-cupy         Run CuPy integration tests.
tests             Run test suite.

[lucascolley]

I would also be keen to know if this breaks anything that someone has set up in their IDE. That might be true if the IDE features require the build dir to be contained in the root dir.

[dschmitz89]

@lucascolley Thanks for working on this infra, highly appreciated. One thing that would really improve the DX for me personally would be the possibility to invoke individual tests via pixi. How complex would that be to implement?