Client interface documentation

Related to the engine/client interface work serving (#45 and the fuller design coming from that), we would benefit from clear/browsable documentation of the interface. [TypeDoc](https://typedoc.org/) is probably a good starting point (and it's incorporated in my client implementation working branch). We should also clarify as part of the README that this (at least) is a specific area of documentation we consider a user-facing requirement/deliverable with ongoing maintenance expectations[^1].

[^1]: Even if that maintenance is just ensuring doc generation works and continues to meet our expectations when generated.

Start by reviewing the engine/client interface work in #45 and the fuller design referenced there, then inspect the client implementation working branch and existing README. Evaluate TypeDoc as the documentation approach for the interface and document the user-facing requirement and maintenance expectations in the README. Done when the interface documentation is browsable and its generation remains maintainable.

Written by the indexing model from the issue text.