Parallelize Base check probes

[codeforester]

Summary

• Run independent Base environment probes concurrently inside the shared check collector.
• Keep Homebrew PATH refresh in the parent shell and merge probe results in the existing deterministic output order.
• Add BATS coverage proving text and JSON ordering while probes overlap.

Validation

• bats cli/bash/commands/basectl/tests/check.bats
• bats cli/bash/commands/basectl/tests/doctor.bats
• env -u BASE_HOME ./bin/base-test
• bash -n cli/bash/commands/basectl/subcommands/setup_common.sh cli/bash/commands/basectl/tests/check.bats cli/bash/commands/basectl/tests/setup_helpers.bash
• git diff --check

Closes #510