feat(ci): reusable FastLED/fbuild/setup composite action (closes #101)

[zackees]

Summary

Ships a composite GitHub Action that lets consumer CI pipelines wire fbuild + cache in one step:

```yaml

• uses: FastLED/fbuild/.github/actions/setup@main
  with:
  cache-key-extra: ${{ hashFiles('platformio.ini') }}
• run: fbuild build examples/Blink -e esp32dev
  ```

What it does

1. `actions/setup-python` with a pinnable Python version (default 3.12).
2. Resolves `FBUILD_CACHE_DIR` to `$RUNNER_TEMP/fbuild-cache` and exports via `$GITHUB_ENV` so every later step inherits it.
3. `pip install fbuild` (pinnable via `fbuild-version` input).
4. `actions/cache@v4` restore/save keyed on OS/arch + cache-version + cache-key-extra + restore-key fallback.

Redirecting to `$RUNNER_TEMP` sidesteps the `~/.fbuild/*/daemon/` gotcha from the cache doc — ephemeral daemon state never ends up inside the cached tree.

Outputs

• `cache-hit` — bool, so consumers can assert warm-build behavior in a smoke step.
• `cache-dir` — resolved path for diagnostics.

Doc update

`docs/CI_CACHING.md` TL;DR now recommends the action first; the raw `actions/cache` snippet is preserved below for consumers who prefer zero action dependencies.

Follow-up

FastLED repo will switch its workflows to use this action in a companion PR (tracked in #101's "Follow-up" section).

Test plan

• YAML validates (GitHub schema).
• README documents inputs, outputs, and a full matrix example with smoke-test cache assertion.
• End-to-end verification happens implicitly when FastLED's workflows adopt it — flagged in feat(ci): ship reusable GitHub Action for fbuild install + cache setup (external consumers) #101.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: bb64f1bf-63a2-4648-8b3b-92bfa37dd30f

📥 Commits

Reviewing files that changed from the base of the PR and between 0b582be and 1fb6807.

📒 Files selected for processing (3)

• .github/actions/setup/README.md
• .github/actions/setup/action.yml
• docs/CI_CACHING.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/101-setup-action

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