Ship a rainix-copy-artifacts.yaml reusable workflow

[thedavidmeister]

Sibling to #168 (shared Solidity library). Both rain.math.float and rain.interpreter (rainlang) currently maintain their own .github/workflows/copy-artifacts.yaml that rebuilds Solidity, extracts the deterministic ABI subset, and asserts git diff --exit-code. The structure is identical except for:

• Whether a meta-build prelude runs first (rainlang needs it, float doesn't).
• The contract list (lives in the repo's own script/lib/LibCopyArtifacts.sol).
• The committed path (crates/float/abi/ vs crates/abi/).

What to ship

rainix-copy-artifacts.yaml reusable workflow that:

1. Checks out the repo.
2. Installs Nix + cache.
3. Runs soldeer install (gated on soldeer.lock existing).
4. Optionally runs a caller-provided prelude command (input prelude:, default empty).
5. Compiles Solidity.
6. Runs the caller's script/CopyArtifacts.sol (which uses the shared LibCopyArtifacts from Ship a shared LibCopyArtifacts in rainix #168).
7. Formats.
8. Asserts git diff --exit-code.

Caller usage shrinks to:

name: copy-artifacts
on: [push]
jobs:
  copy-artifacts:
    uses: rainlanguage/rainix/.github/workflows/rainix-copy-artifacts.yaml@main
    with:
      prelude: <meta-prelude-name>

Migration

Land #168 (shared Lib) and this reusable together, then port both rain.math.float and rain.interpreter to consume them. Leaving either repo on a local copy defeats the purpose.