Contributor lookups for composite and formula_tgv phenotypes

The one place a contributor of a composite phenotype — the individual itself, its dam or sire, or its group-mates — becomes a per-individual value. Both .assemble_composite_tgv() (phenotype_components) and .build_tgv_env() (formula_tgv) read through these helpers, and .ap_materialize_tgvs() uses the same group lookup to decide whose genetic values to compute first. Every lookup reads ind_tgv: the total (ind_tgv_total) by default, or the listed components. Individual ids never appear in SQL text: every lookup joins a registered view.

Group semantics (SGE / Bijma): a focal's group-mates are the other individuals with the same value of group_column in group_table; the aggregate is over the mates that have a genetic value; a focal with no mates gets 0; a focal whose group value is NULL gets NA (a missing component). group_table must have exactly one row per focal individual.