Resolve the basket-allocation decision for this strategy (progressive disclosure).

A basket (len(instruments) >= 2) must declare how its capital splits across instruments before it compiles. This thin, strategy-scoped tool reports current status:

• not_applicable — a single-instrument leaf has no siblings to split.
• required_unset — a basket with no explicit allocation yet; returns the recommended call (equal_weight) and the alternatives.
• set — a basket whose allocation is already chosen; echoes it.

For the full set-allocation surface (score, top_n, rebalance rules) call builder_set_allocation.