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

crates/aura-registry/src/lib.rs

@@ -968,7 +968,9 @@ impl fmt::Display for RegistryError {
                 aura_core::DocGateFault::Empty => write!(
                     f,
                     "blueprint: composite `{name}` carries no doc — a registered \
-                     composite describes itself (C29); set .doc(\"...\") before register"
+                     composite describes itself (C29); add a doc line (builder \
+                     .doc(\"...\"), blueprint \"doc\" member, or op-script \
+                     {{\"op\":\"doc\"}}) before register"
                 ),
                 aura_core::DocGateFault::RestatesName => write!(
                     f,