Decide conditional-mandatory field mechanism in LinkML

Some v2 fields should only become mandatory depending on the experiment type being described, for example, `target_gene` is only mandatory for metabarcoding, not for exome sequencing.

Need to confirm that LinkML can express this kind of "conditional mandatory" logic, and agree on the mechanism before the v2 schema is finalised.

Directly related to GitHub Issue #20 ("Mandatory fields: change to 'importance level'?").

Start by reviewing the conditional-mandatory example for `target_gene` and the discussion in related Issue #20. Investigate whether LinkML can express requirements that depend on experiment type, then document the agreed mechanism. Done means the mechanism is confirmed and suitable for the v2 schema before it is finalised.

Written by the indexing model from the issue text.