feat: the select-rule refusal enumerates the accepted roster (closes #301)

An unknown select rule in a process document now refuses with the
accepted vocabulary beside the quoted offending value:
  block std::sweep: unknown select rule "plateau" (accepted: argmax |
  plateau:mean | plateau:worst)
mirroring the roster-naming metric-refusal shape. The roster is derived
from slot_kind_label(SlotKind::SelectRule) — CLI describe output and
the document refusal stay in sync by construction — and a companion
test pins the "select rule: " prefix that derivation trims, so a
label-wording drift breaks a test instead of silently degrading the
prose. No alias-hint sentence: the enumeration alone rescues the
bare-plateau paste case (fieldtest df_3), and the document schema stays
strict per #300 F7.

crates/aura-research/src/lib.rs

@@ -295,10 +295,17 @@ fn select_from(
 ) -> Result<SelectRule, String> {
     let s = require_str(m, "select", block)?;
     // SelectRule's own derived Deserialize (which carries the #[serde(rename)]
-    // wire strings) is the type oracle, mirroring kind_tag/scalar_from_bare:
-    // no second hardcoded copy of the three select-rule wire strings.
-    serde_json::from_value(serde_json::Value::String(s.clone()))
-        .map_err(|_| format!("block {block}: unknown select rule \"{s}\""))
+    // wire strings) is the type oracle for *parsing*, mirroring
+    // kind_tag/scalar_from_bare. The refusal's roster text below is sourced
+    // from slot_kind_label(SlotKind::SelectRule) rather than a second
+    // hardcoded copy of the three wire strings: CLI describe output and the
+    // document refusal stay in sync by construction (add a SelectRule
+    // variant, update one string).
+    serde_json::from_value(serde_json::Value::String(s.clone())).map_err(|_| {
+        let roster =
+            slot_kind_label(SlotKind::SelectRule).trim_start_matches("select rule: ");
+        format!("block {block}: unknown select rule \"{s}\" (accepted: {roster})")
+    })
 }
 
 fn mode_from(
@@ -1413,6 +1420,20 @@ mod tests {
         assert!(!msg.contains("alias"), "refusal grew an alias hint: {msg}");
     }
 
+    /// The refusal derives its roster from slot_kind_label by trimming the
+    /// "select rule: " prefix. That trim is a positional contract: if the
+    /// label's wording ever drifts, trim_start_matches degrades to a no-op
+    /// and the refusal grows a redundant prefix without any test noticing.
+    /// This pin makes that drift loud.
+    #[test]
+    fn select_rule_label_keeps_the_prefix_the_refusal_trims() {
+        assert!(
+            slot_kind_label(SlotKind::SelectRule).starts_with("select rule: "),
+            "slot_kind_label(SelectRule) no longer starts with \"select rule: \" — \
+             update select_from's roster derivation alongside the label"
+        );
+    }
+
     pub(crate) const CAMPAIGN_FIXTURE: &str = r#"{
       "format_version": 1,
       "kind": "campaign",