Fix bash 3.2 compatibility for macOS install

[ai-tesi]

Summary

• Skip loading doctor.sh on bash < 4 to fix curl | bash install on macOS

Problem

macOS ships with bash 3.2 (from 2007). When running:

curl -sSL https://raw.githubusercontent.com/mbailey/metool/master/install.sh | bash

The installer fails with:

declare: -g: invalid option

This is because lib/doctor.sh uses declare -g which requires bash 4+.

Solution

Conditionally load doctor.sh only when bash 4+ is available:

if [[ "${BASH_VERSINFO[0]}" -ge 4 ]]; then
  source "${MT_ROOT}/lib/doctor.sh"
fi

Trade-offs

• mt doctor command won't be available on bash 3.2
• All other functionality works normally
• Once Homebrew bash is installed, mt doctor becomes available

Testing

Tested on fresh macOS Tahoe VM (arm64) with bash 3.2.57:

• Before: install.sh fails immediately with declare error
• After: install.sh completes successfully

Alternative Considered

Full bash 3.2 compatibility would require rewriting:

• declare -g → plain assignments (easy)
• declare -A → parallel arrays (hard)
• mapfile → while-read loops (medium)
• ${var^^} → tr calls (medium)

This minimal fix unblocks installs without major refactoring.

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