What we need is a programming language that defines the diff to be applied upon the existing codebase to the same degree of unambiguity as the codebase itself.

That is, in the same way that event sourcing materializes a state from a series of change events, this language needs to materialize a codebase from a series of "modification instructions". Different models may materialize a different codebase using the same series of instructions (like compilers), or say different "environmental factors" (e.g. the database or cloud provider that's available). It's as if the codebase itself is no longer the important artifact, the sequence of prompts is. You would also use this sequence of prompts to generate a testing suite completely independent of the codebase.

Please don't pollute HN with LLM word salad.