Products - Compute Technical Fields (template from Variant)

This module is a technical helper for developers. It does not add any new screens, menus, or buttons. Instead, it provides two methods on the product template model (product.template) that other modules can use to compute or propagate field values between a product template and its variants.

The first method, _compute_template_field_from_variant_field, sets a field on the template based on the value of the same field on its variants. If the template has exactly one variant, the template field is set to that variant's value. If there are no variants, it tries again ignoring archived variants. If there are multiple variants, it sets the field to a default value (which can be specified). This is useful for fields like barcode, weight, or volume that are often the same across a single-variant product.

The second method, _set_product_variant_field, does the opposite: it propagates a field value from the template to its unique variant. If the template has exactly one variant, that variant's field is set to the template's value. If there are no active variants but one archived variant, that archived variant is updated.

These methods are meant to be called by other modules to avoid duplicating logic. They do not change any behaviour on their own; they only provide reusable functions. The module depends on the standard product module and extends the product.template model.