Add totally_ordered property to QuerySet to check for deterministic ordering

[VIZZARD-X]

Currently, QuerySet.ordered returns True if any ordering is applied, but this does not guarantee deterministic results. For example, ordering by a non-unique field (like pub_date) is considered "ordered", but the relative order of records with the same date is undefined and database-dependent.

This lack of determinism causes issues in pagination and serialization (specifically for natural keys), where a stable sort order is required to ensure consistency.

I propose adding a property (tentatively totally_ordered) to QuerySet that checks if the applied ordering is sufficient to guarantee a stable sort. This would verify if the order clauses include a unique field (like the primary key) or a set of fields covered by a unique constraint.

Note: Similar logic already exists in django.contrib.admin.views.main.ChangeList.get_ordering. The plan is to extract that logic, make it public/reusable on QuerySet, and document it.

Context: This feature was identified as a requirement for fixing #36750 (deterministic M2M serialization).

[Jacob Walls]

Accepting without taking a trip to django/new-features because it's a requirement for a pending ticket (#36750) that could have been worked on as Refs #36750, but we decided to explode into a second ticket for clearer discussion.

Tentatively assigning to the author of #36750, who I think volunteered to push it forward.

[VIZZARD-X]

PR opened: ​https://github.com/django/django/pull/20518