meaning - What conjugation of 願う is 願わくば, and what does it mean here?

This is my first post here, I'm sorry if I didn't follow any formatting or posting rules.

Original sentence, from a character's monologue:

願わくばこの時間が少しでも長く続くことを。。。

The English translation I have (not mine or official) is

I wish this moment could last even a little bit longer.....

I was quite confused by this conjugation, so I did some research on this site, and read this answer on how conditionals can be represented as 連用形 + あれば. So is 願わくば a contraction of 願わなくあれば / 願わなければ ?

But the thing I'm more confused about is, why is the negative conditional form used here in the first place? In the plain conditional, this looks something like "If I had to wish for something, it would be for this moment to continue for a little longer", but why the seemingly negative form? (Or is it even negative to begin with?)

Sorry for the double triple-loaded question, but one more thing: is this contraction of the conditional form commonly used? (Like in informal contexts?)

Edit: In case you're wondering why I made these somewhat far-fetched guesses: I guessed negative because of the わ (願う->願わない), and guessed conditional because of the ば (願う->願えば). Sorry >.<

Thank you for your time.

Answer

This 願わくば is a fixed expression fossilized long ago, and you just have to memorize it without thinking about it too much. It's a literary expression that corresponds to "Hopefully, ..." used as a sentence adverb.

As pointed out in the comment, this is related to ク語法, a grammatical feature which had already dropped out of use more than 1000 years ago. It was originally 願わくは (nominalized 願う + topic marker は = "What I hope is ..."). 願わくは is still used, but I hear 願わくば more often.

Here are some words and phrases which are etymologically related to ク語法. It's not really worth analyzing them too much unless you want to be an expert of old Japanese grammar.