Design the layer-source model: roles, precedence, and configuration #468

There is no architecture document for writ's layer model -- it lives in plan documents and in the code.
This task writes it, and settles: how a source is named, which role it fills, where that is configured, and
the precedence rule when two sources provide the same path.

The layer tree is packaging and contributes no configuration values; the config tree is a separate
model rolled up on source, scope and builtin. The document must keep them apart rather than unify them.

Epic: #466 · Feature: #467