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

README.md

@@ -119,8 +119,9 @@ aura sweep crossover.bp.json --list-axes
 ```
 
 The op kinds are `source`, `input`, `add`, `feed`, `connect`, `expose`, `tap`,
-and `gang` (`tap` declares a recorded measurement point on an interior wire —
-see the authoring guide). See
+`gang`, and `doc` (`tap` declares a recorded measurement point on an interior
+wire; `doc` declares the composite's one-line meaning, required at register —
+C29; see the authoring guide). See
 `aura graph introspect --node <T>` for a type's exact ports and the op-script
 grammar in the design ledger for the full semantics.