This requires a few pieces:
* Make node tags use `Fin n` intead of natural numbers. This makes
it possible to build a finite lattice over AST nodes, and also
ensure automatic, total indexing from CFG nodes into the AST that
created them. For this, use the elaborator to derive the ordering
statements etc. where possible.
* Adjust the forward framework to enable proofs that don't just state
correctness on the environment, but also on an arbitrary additional
state accumulated from traversing the trace.
* State the reaching definition analysis's correctness in terms
of this new framework.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
