Decide the runtime module name and set it up as a separately tagged module

Part of #37. Came out of an ideation session, not a scoped request.

The runtime that generated projects compile against moves into its own module with its own `go.mod`, tagged on its own line (`/vX.Y.Z`) so it can be released independently of the CLI. The name is an open decision: `core` and `lib` were both raised and neither was picked.

This is the piece that reverses the current single-module design, so it should also update the distribution section of `roadmap.md`.

Start by reviewing the current single-module layout and the distribution section of roadmap.md. Resolve the open runtime module name decision, then define the separate go.mod and independent tag naming; done means the runtime can be released independently of the CLI and roadmap.md reflects the design.

Written by the indexing model from the issue text.