Migrate conda CI from conda-build to rattler-build

🤖 AI text below 🤖

The Conda workflow uses conda-build with a conda.recipe/meta.yaml. This is showing its age: conda-build is now a conda plugin, and recent setup-miniconda default changes keep breaking the build (see #208 for the latest patch).

We should move to rattler-build, the maintained conda package builder from the prefix.dev / conda-forge folks. It is faster, does not need a full miniconda + conda-build install, and has an official GitHub Action.

Rough scope:

• Add a recipe/recipe.yaml in the rattler-build schema (replacing conda.recipe/meta.yaml).
• Replace the setup-miniconda + conda build steps in .github/workflows/conda.yml with prefix-dev/rattler-build-action (or a pixi-based build).
• Keep the Python matrix and the pytest tests check.
• Once green, drop the old conda.recipe/ and the conda-build-specific steps.

This is a template repo, so the recipe doubles as an example for downstream users — worth keeping it clean and current.

References:

• rattler-build: https://rattler.build
• action: https://github.com/prefix-dev/rattler-build-action

Read .github/workflows/conda.yml and conda.recipe/meta.yaml first, then consult the rattler-build recipe schema and prefix-dev/rattler-build-action documentation. Done means recipe/recipe.yaml builds successfully across the existing Python matrix, pytest tests still runs, and the old conda.recipe/ and conda-build-specific workflow steps are removed.