This is implemented as an overloaded function,
`assert_baklava_phase_1_invariants()`, with variants for `Action`, `Layer`, and
`Strip`.
The invariants asserted are those that are specific to phase 1 and which will
later be lifted as more features (such as animation layers, additional strip
types, etc.) are added. The intention is for this to serve as a kind of todo
marker for later phases *and* to help ensure that the phase-1 invariants
currently hold at runtime.
In addition to adding the overloaded function itself, this commit also uses it
in a couple of places in the keyframing code. Upcoming work on the keyframing
code will be using it more.
This is based on a discussion with @dr.sybren.
Pull Request: https://projects.blender.org/blender/blender/pulls/123830
c83727f0bd