fix(docs, registry): fieldtest follow-ups -- guide sync, doc-op documentation, surface-neutral refusal

Three of the four immediate fieldtest findings for the self-description
cycle:

- authoring-guide S0: the worked Scale literal gains the required
  NodeSchema.doc line (+ the C29 comment) -- the shipped example was a
  hard E0063 for anyone pasting it (the exact #314 teaching-failure
  class, caught from the consumer seat).
- The op table becomes "The nine ops": the C29 doc op is documented
  with its JSON shape, the register requirement, the at-most-once rule
  and the gate; the README op-kinds list names it too. The fieldtester
  had to guess the shape from commit bodies -- now it is on the public
  surface.
- The store-gate refusal's fix hint is surface-neutral: builder
  .doc(...), blueprint "doc" member, or op-script {"op":"doc"} --
  previously it prescribed the Rust builder to a JSON author.

Gates: registry, graph_construct (63) and project_load (14) suites
green; refusal-prose pins unchanged (pinned substrings untouched).

refs #316

docs/authoring-guide.md

@@ -134,6 +134,9 @@ impl Scale {
                 }],
                 output: vec![FieldSpec { name: "value".into(), kind: ScalarKind::F64 }],
                 params: vec![ParamSpec { name: "factor".into(), kind: ScalarKind::F64 }],
+                // C29: every vocabulary entry ships a one-line meaning — the
+                // doc field is required (E0063 without it) and gated at load.
+                doc: "scalar gain: emits the input times the factor param",
             },
             |p| Box::new(Scale::new(p[0].f64())),
         )
@@ -217,10 +220,11 @@ $ aura graph build < smacross.json > blueprint.json
 Nodes are referenced by an **identifier** (given by `add`, see below); ports
 are dotted `<identifier>.<port>` on both sides of a wire.
 
-### The eight ops
+### The nine ops
 
 | op | JSON shape | does |
 |---|---|---|
+| `doc` | `{"op":"doc","text":<str>}` | declare the composite's one-line meaning (C29) — the op-script twin of the Rust builder's `.doc(...)`. Required before `aura graph register` accepts the product (the store refuses a doc-less composite); at most one per script (a second refuses: `a doc op may appear at most once`). The text is gated: empty or merely restating the composite's name refuses at register. |
 | `source` | `{"op":"source","role":<str>,"kind":<ScalarKind>}` | reserve a bound root **source** role of `kind` — a real input the harness feeds (e.g. `"price"`). |
 | `input` | `{"op":"input","role":<str>}` | reserve an open root **input** role (kind inferred from the slots it feeds) — for a fragment meant to be wired by an *enclosing* graph. A standalone document built with `aura graph build` finalizes as a closed root, so an `input` role that is never bound refuses at the end: `finalize: root input role <name> is unbound`. |
 | `add` | `{"op":"add","type":<TypeId>,"name":<str>?,"bind":{<param>:<Scalar>}?}` | instantiate a node of a type in the closed vocabulary (`aura graph introspect --vocabulary`) — see §0 below for how a type gets into that vocabulary in the first place. `name` becomes the node's identifier for later ops (default: the type's own lowercase label — two unnamed nodes of the same type then collide). `bind` sets zero or more of its params. |