A primary-key column never takes its default

Carry the :default_value: guard into the primary-key branch of the domain template, so a key column can state an initialiser like every other column can.

cpp_domain_type_class.hpp.mustache emits the two shapes differently:

{{#primary_key.columns}}
    {{{cpp_type}}} {{column}};                                       <- no initialiser
{{/primary_key.columns}}
{{#natural_keys}}
    {{{cpp_type}}} {{column}}{{#default_value}} = {{{default_value}}}{{/default_value}};
{{/natural_keys}}

So :default_value: 0 on a primary-key column is silently ignored. Every precedent that works — credit_simulation_config.seed, bond_formula_based_leg_data.fixing_days — is a non-key column.

The consequence is the opposite of what a key needs. An ordinal in a composite key is the field a writer is most likely to forget, because it is metadata about the row rather than data from the document; with no initialiser the forgotten value is an indeterminate int, so the failure is undefined rather than the constraint's refusal.

Found while giving instrument_strikes.sequence_number a default of zero. The model gained the property, regeneration ignored it, and the member stayed bare. Changing this affects every generated key in every component, so it wants its own unit and a regeneration sweep.

• Carry the rates family's fields across the ORE boundary — where it was found.
• projects/ores.codegen/library/templates/cpp_domain_type_class.hpp.mustache, the two branches.