add a compound_id which can compose a feature identifier along with with module identifiers into a single id

[hellkite500]

Introduce an engine-level compound identity for BMI formulation instances. Designed as a unique per-formulation key.

Default composition for a single-BMI formulation:

"<this->id>:<model_type_name>"
e.g. "cat-1:bmi_c_cfe"

For a Bmi_Multi_Formulation submodule, the parent injects a three-part compound via set_compound_id() inside init_nested_module, BEFORE the submodule's create_formulation() runs — so any engine-level consumer that reads compound_id() during submodule construction sees the full key:

"<this->id>:<submodule-mtn>:<multi-mtn>"
e.g. "cat-1.0:bmi_c_cfe:bmi_multi_noahowp_cfe"

The submodule's this->id is already <catchment>.<index> from Bmi_Multi_Formulation's existing identifier construction; we append its own model_type_name (peeked from the config map so it's available pre-create_formulation) and the enclosing multi's model_type_name.

this->id is not mutated — it stays the feature identifier used by current code paths (output naming, forcing lookup, logs). The compound is a read-only view of the formulation's place in the realization, built from fields already in hand.

This is used in #957, and was extracted as an independent feature from that effort.

Additions

• a formulation compound_id_ member and getter/setters

Checklist

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