Upstream a regen-derived-artifacts-and-assert-clean-tree reusable workflow

[thedavidmeister]

A generic "regen-all-derived-artifacts-and-assert-clean-tree" reusable workflow keeps surfacing as inline-duplicated per repo. Each consumer's git-clean / copy-artifacts workflow ends up doing the same orchestration:

1. Checkout.
2. Nix install + cache.
3. soldeer install (gated on soldeer.lock).
4. Run a repo-specific meta-build prelude (a nix run .#<task> that invokes the rain CLI to produce meta/*.rain.meta).
5. Run script/BuildPointers.sol (where present) to regenerate src/generated/*.pointers.sol.
6. Optional: run script/CopyArtifacts.sol (where present) to regenerate crates/*/abi/*.json.
7. Run forge fmt.
8. Assert git diff --exit-code.

Current duplicates:

• rain.math.float/.github/workflows/copy-artifacts.yaml (no meta, has pointers + abi)
• rainlang/.github/workflows/copy-artifacts.yaml (meta + pointers + abi)
• rain.flare/.github/workflows/git-clean.yaml (meta + pointers, no abi) — about to land

rainix-build-pointers.yaml covers step 5 only and assumes no prelude.

What to ship

rainix-regen-check.yaml (name TBD) reusable workflow with inputs:

inputs:
  prelude:
    type: string
    required: false
    default: ""
    description: Optional name of a `nix run .#<name>` task to run before the build (e.g. for meta regen via the rain CLI).
  pointers:
    type: boolean
    default: true
  copy-artifacts:
    type: boolean
    default: false

Consumer reduces to a 6-line wrapper.

Relationship to existing issues

• Ship a shared LibCopyArtifacts in rainix #168 (shared LibCopyArtifacts) — separate layer, the shared library used by the abi-copy script.
• Ship a rainix-copy-artifacts.yaml reusable workflow #169 (rainix-copy-artifacts.yaml) — abi-copy specific. This issue could absorb Ship a rainix-copy-artifacts.yaml reusable workflow #169 by making the abi step optional in the unified workflow. Either way they should ship together.

Migration

Land the reusable, then port float, rainlang, and rain.flare from their local copies. Leaving any of them on a local copy defeats the purpose.