Document basectl check parallelism plan

docs/README.md

@@ -35,5 +35,7 @@ reference. The filename should answer "what is this about?"
   installers should use Base without moving product-specific logic into Base.
 - [`basectl onboard`](basectl-onboard.md) captures the guided setup experience
   and its relationship to project installers.
+- [`basectl check` parallelism](check-parallelism.md) records the evaluation and
+  implementation constraints for parallel check probes.
 - [base_cli Runtime Package](base-cli.md) describes the Python CLI foundation
   used by Base and Base-supported project CLIs.

docs/check-parallelism.md

@@ -0,0 +1,78 @@
+# basectl check parallelism
+
+`basectl check` is called frequently enough that reducing wall time matters, but
+the command also has to stay easy to reason about. The output order is part of
+the user experience and the JSON order is useful to automation, so the command
+should not print directly from background jobs.
+
+## Decision
+
+Parallelize only independent probes, collect their results, and render them in
+the existing deterministic order.
+
+Good candidates:
+
+- Homebrew presence and path discovery
+- Xcode Command Line Tools presence
+- Homebrew Python formula presence
+- Base virtual environment presence
+- Base bootstrap Python package checks
+
+Do not parallelize steps that mutate process state or depend on an earlier
+probe's side effect. In particular, `setup_refresh_brew_path` modifies `PATH`
+and should remain in the parent shell after Homebrew discovery succeeds.
+
+## Recommended Shape
+
+Introduce small result-producing helpers instead of backgrounding the current
+logging flow directly:
+
+```bash
+setup_check_homebrew_probe >"$tmpdir/homebrew" &
+setup_check_xcode_probe >"$tmpdir/xcode" &
+setup_check_python_probe >"$tmpdir/python" &
+setup_check_venv_probe >"$tmpdir/venv" &
+setup_check_python_package_probe "$pyyaml_package" >"$tmpdir/pyyaml" &
+setup_check_python_package_probe "$click_package" >"$tmpdir/click" &
+wait
+```
+
+Each probe should write structured shell-safe data such as:
+
+```text
+ok=true
+message=Homebrew is installed.
+recovery=
+```
+
+The parent should then read those files and emit text or JSON in the current
+order:
+
+1. Homebrew
+2. Xcode Command Line Tools
+3. Python formula
+4. Base virtual environment
+5. PyYAML
+6. click
+7. developer prerequisite checks, when `--dev` is set
+8. project artifact checks, when a project is supplied
+
+## Constraints
+
+- Preserve `basectl check --format json` schema and ordering.
+- Preserve the text output order.
+- Keep `setup_clear_run_state` before probe collection.
+- Keep `setup_require_macos` before probe collection.
+- Avoid background jobs for project artifact checks until the Python layer has
+  an explicit concurrent check API.
+- Avoid background jobs for developer prerequisite checks until `base_dev`
+  exposes a result-only mode that can be merged deterministically.
+
+## Follow-Up
+
+The implementation should be a dedicated performance PR with focused tests for:
+
+- deterministic text output order
+- deterministic JSON output order
+- missing Homebrew with other probes still reported
+- package checks not failing spuriously when the venv is absent