fix: pixi-build-r recommended dependencies

[fhaefele]

pixi-build-r treated R recommended packages as included in r-base. However, conda-forge builds r-base with [--with-recommended-packages=no] (https://github.com/conda-forge/r-base-feedstock/blob/main/recipe/build-r-base.sh#L208) and provides these packages as separate r-* artifacts.

This caused source builds to fail when their DESCRIPTION imported recommended packages. For example, NLRAA requires boot, MASS, Matrix, mgcv, and nlme. R CMD INSTALL failed because none were present in the build environment.

This change keeps only Priority: base packages in R_BUILTIN_PACKAGES. Recommended packages now become normal host and run requirements:

boot   → r-boot
MASS   → r-mass
Matrix → r-matrix
mgcv   → r-mgcv
nlme   → r-nlme

Base packages such as stats remain excluded.

Minimal reproduction:

[workspace]
name = "nlraa-reproduction"
channels = ["conda-forge"]
platforms = ["linux-64"]
preview = ["pixi-build"]

[dependencies]
r-base = ">=4.5,<4.6"

r-nlraa = {
  git = "https://github.com/femiguez/nlraa",
  rev = "d6afbf815a423077286821acc97ce44184ecb039",
  package.build.backend = {
    name = "pixi-build-r",
    version = "*",
    channels = [
      "https://prefix.dev/pixi-build-backends",
      "conda-forge",
    ],
  },
}

Run:

pixi install

Before this fix:

ERROR: dependencies ‘boot’, ‘MASS’, ‘Matrix’, ‘mgcv’, ‘nlme’ are not available

After this fix, Pixi resolves the separate conda-forge packages, builds NLRAA, and records the complete dependency graph in pixi.lock.

How Has This Been Tested?

Added a regression test using NLRAA's dependencies. It verifies that:

• Recommended packages become r-* host requirements.
• Recommended packages become r-* run requirements.
• Base packages such as stats remain excluded.

Commands:

pixi run cargo test -p pixi-build-r
pixi run lint

Results:

• All 39 pixi-build-r tests passed.
• Full repository lint passed.
• Built NLRAA 1.9.11 from the pinned Git commit using the fixed local backend.
• Verified library(nlraa) succeeds.
• Verified r-boot, r-mass, r-matrix, r-mgcv, and r-nlme are resolved from conda-forge and recorded in pixi.lock.

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.

Tools: Codex, gpt-5.6-sol

Prompt:

Fix pixi-build-r so recommended R packages distributed separately by conda-
forge are added as host and run dependencies. Add regression coverage and
verify the change using NLRAA.

Checklist:

• I have performed a self-review of my own code
• I have added sufficient tests to cover my changes.