Declare run-exports to consolidate downstream consumers' duplicate pixi.toml entries

[joshkamm]

Downstream consumers (ZEST, XCtera) currently need to list slatergpu in both [package.host-dependencies] and [dependencies] of their pixi.toml. Switching to a feature branch or local checkout for codevelopment requires editing two places — easy to forget one. This pain applies even when the source is a git-source pull rather than a published conda package, since pixi-build still processes the package metadata.

Goal

Investigate and adopt run-exports so downstream consumers list slatergpu in only one place.

Scope notes

Small ergonomics improvement, not blocking any other work. Safe to defer or close as wontfix if pixi-build doesn't support run-exports cleanly.

Investigation steps

1. Confirm pixi-build-cmake (currently >=0.3.6,<=0.3.8 per consumer pixi.toml) supports run-exports declaration. Reference: pixi-build docs / repo issues. If unsupported, file an upstream issue with pixi and document the two-place pattern in the meantime.
2. If supported, declare in SlaterGPU/pixi.toml (likely [package.run-exports] or similar — confirm syntax).
3. Update at least one downstream pixi.toml (probably XCtera in a follow-up issue C PR) to drop the duplicate listing and verify the build still works.

Legacy workflow impact

Low. Paul's Makefile and manual CMake builds don't go through pixi-build packaging.

Acceptance

• SlaterGPU declares run-exports correctly (or this issue documents that pixi-build doesn't yet support it and links the upstream issue).
• A test consumer (XCtera or a scratch project) successfully omits the duplicate slatergpu entry from its pixi.toml.

Related

• Enable libcint 4-center 1-electron overlap integrals (WITH_4C1E) #82 — if the pixi-recipe path is chosen for libcint there, the same run-exports pattern applies. Coordinate sequencing.
• Part of a broader dependency-infrastructure effort tracked in ZimmermanGroup/XCtera (umbrella tracking issue will be linked once filed).

[joshkamm]

Investigation outcome: blocked upstream

pixi-build-cmake (the build backend used by slatergpu and its downstream consumers) does not currently support a run-exports declaration on the producer side.

• pixi-build-cmake's CMakeBackendConfig (prefix-dev/pixi-build-backends/crates/pixi-build-cmake/src/config.rs) uses #[serde(deny_unknown_fields)] and exposes exactly five options: extra-args, env, debug-dir, extra-input-globs, compilers. No run-exports field. gh search code "run_exports" across the prefix-dev/pixi-build-backends repo returns zero results.
• Pixi docs (dependency_types) describe run-exports only from the consumer side. No author-side syntax is documented.
• Upstream tracking issue: prefix-dev/pixi#4021 — open, with the exact proposed syntax we'd want:

  [package.run-exports.weak]
  slatergpu = { path = "." }

Taking the "documented & deferred" branch of this issue's acceptance criteria. Keeping this issue open as a watch-item rather than closing as wontfix, so it stays visible in the backlog. A 👍 reaction + a comment posted on the upstream issue supplies the passive notification trigger.

When upstream lands, the change here will be a small addition to pixi.toml plus dropping the duplicate slatergpu listing from each consumer's [dependencies] stanza.