Implement parallel Base check probes

[codeforester]

Finding

docs/check-parallelism.md already captures a good design for speeding up basectl check, but the implementation still collects the base environment probes serially in setup_collect_base_check_results.

basectl check is a high-frequency command. It should feel nearly instant when the machine is healthy.

Project fields

• Priority: P2
• Size: M
• Initiative: Performance

Expected outcome

Independent Base environment probes should run in parallel while preserving deterministic text and JSON output order.

Good first targets from the existing design doc:

• Homebrew presence and path discovery
• Xcode Command Line Tools presence
• Homebrew Python formula presence
• Base virtual environment integrity
• Base bootstrap Python package checks

Suggested implementation

• Follow the shape in docs/check-parallelism.md.
• Have each probe write structured result data to a temp file or equivalent result channel.
• Merge results in the current display order before printing text or JSON.
• Keep state-mutating work, especially setup_refresh_brew_path, in the parent shell.
• Do not parallelize profile or project artifact checks until the Python layer has an explicit concurrent diagnostic API.

Validation

• Add BATS coverage for deterministic text output order.
• Add BATS coverage for deterministic JSON output order.
• Test missing Homebrew with other probes still reported.
• Test package checks do not fail spuriously when the venv is absent.
• Run env -u BASE_HOME ./bin/base-test.