test: red for the select-rule refusal enumerating the accepted roster (refs #301)

A process document carrying an unknown select rule refuses correctly
but names no accepted vocabulary; the fieldtest's bare-plateau paste
(df_3) hits an opaque wall. The unit test pins the wanted property at
the site that builds the prose (select_from): the refusal enumerates
argmax, plateau:mean, plateau:worst beside the quoted offending value.
No alias-hint sentence — the enumeration alone rescues the paste case
and the project's refusal prose stays sparse; the document schema stays
strict per #300 F7.

crates/aura-research/src/lib.rs

@@ -1381,6 +1381,38 @@ mod tests {
         assert!(matches!(parse_process("not json"), Err(DocError::NotJson(_))));
     }
 
+    /// #301 (fieldtest cycle-300 df_3): the unknown-select-rule refusal must
+    /// teach the accepted vocabulary, not just refuse — an author who learned
+    /// bare `plateau` at the CLI pastes it into a document and needs the
+    /// roster (argmax | plateau:mean | plateau:worst) in the refusal prose to
+    /// self-serve the fix. The schema stays strict: bare "plateau" remains
+    /// refused in documents (#300 F7 — it is a CLI-only argv alias), and the
+    /// prose stays sparse — the roster only, no alias-bridging sentence.
+    #[test]
+    fn unknown_select_rule_refusal_enumerates_accepted_rules() {
+        let bare_plateau = PROCESS_FIXTURE.replacen("plateau:worst", "plateau", 1);
+        let err =
+            parse_process(&bare_plateau).expect_err("bare plateau stays refused in documents");
+        let DocError::Malformed(msg) = err else {
+            panic!("expected DocError::Malformed, got {err:?}");
+        };
+        // The existing contract survives: the offending value stays named, quoted.
+        assert!(
+            msg.contains("unknown select rule \"plateau\""),
+            "offending value no longer named: {msg}"
+        );
+        // The new behaviour: the refusal enumerates the accepted rules.
+        for accepted in ["argmax", "plateau:mean", "plateau:worst"] {
+            assert!(
+                msg.contains(accepted),
+                "accepted rule {accepted:?} not enumerated in the refusal: {msg}"
+            );
+        }
+        // Precise and sparse: the roster alone rescues the paste case — no
+        // alias-hint sentence bridging plateau -> plateau:mean.
+        assert!(!msg.contains("alias"), "refusal grew an alias hint: {msg}");
+    }
+
     pub(crate) const CAMPAIGN_FIXTURE: &str = r#"{
       "format_version": 1,
       "kind": "campaign",