Plant initial stem diameter is arbitrary

The initial plant stem diameter during recruitment is entirely arbitrary - we need to find some kind of initial DBH and stochiometry from the carbon and nutrients available in the propagule.

We could set that as a trait, but it would be better to work it out from first principles using the T Model.

_Originally posted by @davidorme in https://github.com/ImperialCollegeLondon/virtual_ecosystem/issues/827#issuecomment-3257767672_

Start by locating the plant recruitment code and its current initial stem diameter calculation; the issue names no file or test. Read the T Model assumptions and how propagule carbon and nutrients are represented, then define what a first-principles initial DBH and stoichiometry should produce. Done means recruitment no longer uses an arbitrary diameter and the behavior is covered by relevant tests.

Written by the indexing model from the issue text.