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

README.md

@@ -77,6 +77,8 @@ naming (`--name`), and selection (`--select`) flags.
 
 A blueprint is authored declaratively from a **JSON op-list**: an ordered list
 of by-identifier construction ops replayed against the standard node vocabulary.
+Adding a new node *type* to that vocabulary is Rust, not JSON — see
+`docs/authoring-guide.md`, §0, "Authoring a new node in Rust".
 
 | Command | Purpose |
 |---|---|