docs: teach the Rust node-authoring pattern in the authoring guide (§0)

New top-level section covering the three-part shape every node type
follows — Node impl (lookbacks/eval/label with the C8 warm-up filter),
PrimitiveBuilder recipe (NodeSchema + positional build closure), and
rostering (project-side aura_project! vs std-side roster macro) — with
the aura-new Scale starter as the verbatim worked example and the five
fresh #236 nodes as arity-variant references. Cross-linked from the
op-script vocabulary note, project-layout, and the README.

closes #228

docs/project-layout.md

@@ -81,6 +81,11 @@ Shared nodes are `rlib` dependencies; the hot-reload unit stays the project-side
 `cdylib` that composes them, so editing a shared node still rebuilds and reloads
 the dependent.
 
+Every tier follows the same three-part pattern — a `Node` impl, a
+`PrimitiveBuilder` recipe, and rostering the type id into a vocabulary
+lookup — worked through end to end, with the scaffold's own starter node as
+the example, in [`docs/authoring-guide.md`, §0](authoring-guide.md#0-authoring-a-new-node-in-rust).
+
 ## Authoring happens in Claude Code (contract C17)
 
 aura has no built-in coding-LLM. You author by talking to Claude Code, which