Add macOS compatibility for SHA256 verification in installer

[Copilot]

Summary

• Purpose and scope
• Linked issue
• Testing done

Addresses feedback on PR #4: installer hard-required sha256sum, unavailable on macOS by default.

Changes

• Added compute_sha256() helper: Auto-detects available hash tool in priority order: sha256sum (Linux) → shasum -a 256 (macOS) → openssl dgst -sha256 (fallback)
• Removed sha256sum from required commands: No longer blocks macOS users
• Updated checksum verification: Calls helper instead of direct sha256sum

compute_sha256() {
  local file="$1"
  if command -v sha256sum >/dev/null 2>&1; then
    sha256sum "$file" | awk '{print $1}'
  elif command -v shasum >/dev/null 2>&1; then
    shasum -a 256 "$file" | awk '{print $1}'
  elif command -v openssl >/dev/null 2>&1; then
    openssl dgst -sha256 "$file" | awk '{print $NF}'
  else
    echo "Error: No SHA256 tool found (tried: sha256sum, shasum, openssl)" >&2
    exit 1
  fi
}

Checklist

• Branch based on latest main
• No direct commits to main
• Updated README.md / docs if needed
• Updated CHANGELOG.md for user-facing changes
• Added/updated tests (if applicable)
• For releases: used make release V=X.Y.Z and opened PR from release/vX.Y.Z

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