[spec] adding a "magic number" for gemini files

[Petite Abeille]

> On Dec 26, 2020, at 16:32, cage <cage-dev at twistfold.it> wrote:
> 
> I wonder if  a magic number (three, maybe four  chars?) could be added
> on the first line of each file.

No*. 

We still pay the price for that BOM abomination. No need to double down.

That said, there are other signs, such as link sequences '=> '.

Otherwise, text/gemini is, rightly, identified as text/plain;charset=utf-8;. 

Why do you need more?

* This is my personal opinion, not an authoritative answer :D