Reuse node, ocaml, and rewatch setup between ci.yml and coverage.yml

[cknitt]

Summary

Extract repeated CI environment setup into local composite actions shared by the main CI, coverage, and release workflows.

What changed

• Add a shared Node setup action that:
  • uses the repository’s configured Node version
  • enables Yarn caching
  • configures the public npm registry
  • installs dependencies with yarn install --immutable
• Add a shared OCaml setup action that:
  • restores or creates the OPAM environment
  • supports different compiler switches and dependency flags
  • reconstructs cached OPAM environments on Linux, macOS, and Windows
  • only creates shared caches during push workflows
• Add a shared Rewatch build action that:
  • restores or builds the requested Rust target
  • optionally runs Clippy and unit tests
  • copies the resulting executable into the expected package locations
• Add a shared Linux system-dependency action.
• Reuse these actions from ci.yml and coverage.yml.
• Reuse the Node action from the playground and publishing jobs.

Why

The CI and coverage workflows independently implemented nearly identical Node, Rust, Rewatch, OCaml, and OPAM cache setup. Keeping those copies synchronized was error-prone, particularly around:

• cache keys and paths
• Rust toolchain versions and targets
• OPAM environment restoration
• Windows-specific OPAM configuration
• dependency installation flags

Centralizing this setup gives those workflows one implementation while retaining their intentional differences, such as coverage’s additional development dependencies and native Rewatch build.

Behavior

This is intended as a structural refactoring. Existing build, test, coverage, artifact, and publishing behavior remains unchanged.

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commit: 94707d7

[github-actions]

Developer playground preview: https://rescript-lang.github.io/rescript/dev-playground/?version=pr-8506

[cknitt]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Swish!

Reviewed commit: 94707d726a

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