While view transitions L1 is pretty stable (a minor bug fix/clarification now and then), L2 is getting a bit complex in terms of being a delta spec, with more and more hooks into internal parts of L1.

Proposing that this is the right time to merge L1 into L2, and soft-freeze L1 (meaning, we generally don't change it but can backport if necessary).

/cc @nt1m @emilio @fantasai @vmpstr