What is the very least a Catholic needs to believe about Genesis 2 and 3?

It is hard to imagine that there is one document from which there is a verse by verse breakdown of what is a metaphor and what is a fact in Gn 2-3. I am not discounting that there could be such a document. There is though 7. De charactere historico trium priorum capitum Geneseos, Concerning the historical nature of the first three chapters of Genesis (June 30, 1909) by the Pontifical Biblical Commission in Italian and Latin.

This is the translation: Concerning the Historical Character of the First Three Chapters of Genesis | The Replies of the Pontifical Biblical Commission On questions of Sacred Scripture | Translated by E. F. Sutcliffe, S.J..

For example, quoting

II: Notwithstanding the historical character and form of Genesis, the special connection of the first three chapters with one another and with the following chapters, the manifold testimonies of the Scriptures both of the Old and of the New Testaments, the almost unanimous opinion of the holy Fathers and the traditional view which the people of Israel also has handed on and the Church has always held, may it be taught that: the aforesaid three chapters of Genesis contain not accounts of actual events, accounts, that is, which correspond to objective reality and historical truth, but, either fables derived from the mythologies and cosmogonies of ancient peoples and accommodated by the sacred writer to monotheistic doctrine after the expurgation of any polytheistic error; or allegories and symbols without any foundation in objective reality proposed under the form of history to inculcate religious and philosophical truths; or finally legends in part historical and in part fictitious freely composed with a view to instruction and edification?
Answer: In the negative to both parts.

and

VI: Provided that the literal and historical sense is presupposed, may certain passages in the same chapters, in the light of the example of the holy Fathers and of the Church itself, be wisely and profitably interpreted in an allegorical and prophetic sense?
Answer: In the affirmative.

Please see also:

• "Catechesis on the Book of Genesis" | Pope St. John Paul II (the Great).
• Hexaemeron | New Advent.

Your question seems to be from a perspective of what is taught as historically accurate and what is considered metaphor. I think there are some foundational ideas which need to be dealt with first in order for you to truly understand the answer to your question.

First, in your question there is an implicit assumption about the Catholic Church's relationship to Scripture which is skewed. The Catholic Church does not go through scripture and declare the meaning of every sentence or whether is it historically accurate or metaphor. There are actually very few passages in which the Church would definitively teach a particular meaning. For example, Mark 14:22: While they were eating, he took bread, said the blessing, broke it, and gave it to them, and said, “Take it; this is my body.” This definitive interpretation comes about from the experience of the Church.

It is better to ask if the Church has defined anything in regard to Genesis 1 and 2.

Second, metaphor can be true. Its truth, though, may be harder to comprehend and less precise than the truth of a mathematical statement such as 2 + 2 = 4. I think you meant what is historically true and what is metaphor.

When looking for 'official' Catholic teaching, the Catechism of the Catholic Church (CCC) is the first place to go to. (In the CCC, sections are numbered for easy reference, as in the 289 that follows.)

Here is sampling:

289 Among all the Scriptural texts about creation, the first three chapters of Genesis occupy a unique place. From a literary standpoint these texts may have had diverse sources. The inspired authors have placed them at the beginning of Scripture to express in their solemn language the truths of creation—its origin and its end in God, its order and goodness, the vocation of man, and finally the drama of sin and the hope of salvation. Read in the light of Christ, within the unity of Sacred Scripture and in the living Tradition of the Church, these texts remain the principal source for catechesis on the mysteries of the “beginning”: creation, fall, and promise of salvation.

375 The Church, interpreting the symbolism of biblical language in an authentic way, in the light of the New Testament and Tradition, teaches that our first parents, Adam and Eve, were constituted in an original “state of holiness and justice.” This grace of original holiness was “to share in... divine life.”

390 The account of the fall in Genesis 3 uses figurative language, but affirms a primeval event, a deed that took place at the beginning of the history of man. Revelation gives us the certainty of faith that the whole of human history is marked by the original fault freely committed by our first parents.

You could actually read from 279-481 of the CCC. These have resulted from millennia of contemplating the Scriptures.

I think some other answers will delve into the Papal Encyclical Humani Generis of Pope Pius XII which will deal with Adam and Eve and polygenism (the human race is of different origins). For a betting understanding of what is in this Encyclical, a few other ideas are needed for background.

1. There are different levels of Church teaching. There is a difference between Church teaching and dogmatic Church teaching. A Papal Encyclical is not dogmatic Church teaching, but it can be said to be Church teaching.

2. With regard to polygenism, the definition of human being can be different between a biological view and a philosophical view. For example, from a philosophical view of human being could be a creature with a rational soul, while a biological one would focus on physical traits. So, from a philosophical view not all Homo Sapiens need be humans. An excellent explanation of this can be found here

This reply may not answer your question in the way you want, but I think it provides some necessary context for the answer.