Kafka functions: architecture and design document

Document the Kafka middleware architecture for contributors and maintainers:

- How the middleware fits alongside the existing `http` and `cloudevents` middleware in func-go
- Consumer group management and offset commit strategy (at-least-once)
- Health endpoint implementation (readiness tied to partition assignment)
- Scaffolding system: how `invoke: kafka` maps to templates and generated `main.go`
- Signal handling and graceful shutdown
- Known limitations and future direction (sub-process model for multi-language support)

Start by reading the existing http and cloudevents middleware and tracing the scaffolding entry point for `invoke: kafka` through to the generated `main.go`. Done means a contributor-facing document covers consumer groups and at-least-once commits, readiness and partition assignment, signal handling, graceful shutdown, scaffolding, limitations, and the future sub-process model.

Written by the indexing model from the issue text.