feat(cli): swap the scaffold's starter node for a param-bearing Scale (#183)

The `aura new` scaffold shipped one node, a param-less Identity, so a
downstream author had no public example of a build closure consuming a
bound param — the highest-value gap the project-environment fieldtest found
(F2): a newcomer could copy a param-less node but had to read engine source
to write their first *tunable* one, which the consumer contract forbids.
Swap that single starter node for a Scale node (mirroring the std gain
operator): its schema declares a `factor` ParamSpec and its build closure
reads the bound value (`|p| Box::new(Scale::new(p[0].f64()))`), and the
starter blueprint binds `factor` so `aura run` exercises the bound-param
path with zero hand-editing.

factor is bound to 1.0 — a neutral identity default the newcomer then tunes,
not the only C7-safe value (only |factor|>1 would push the terminal `bias`
output past the bounded [-1,+1] contract; the knob is genuinely read and
multiplied each cycle). The param-reading closure is the copyable template
content #183 delivers; a hypothetical `|_|` regression is caught by the
scaffold unit test's source guard, and tests/project_new.rs re-validates the
swap by building and running the scaffold end to end.

Template-side only (#181): the frozen 0102 demo-project fixture is untouched.

closes #183

crates/aura-cli/src/scaffold.rs

@@ -115,22 +115,24 @@ const LIB_RS: &str = r#"//! __NAME__ — an aura research project: node logic +
 //! this cdylib per invocation and merges it with the std vocabulary.
 
 use aura_core::{
-    Cell, Ctx, FieldSpec, Firing, Node, NodeSchema, PortSpec, PrimitiveBuilder,
+    Cell, Ctx, FieldSpec, Firing, Node, NodeSchema, ParamSpec, PortSpec, PrimitiveBuilder,
     ScalarKind,
 };
 
-/// One-input f64 pass-through. Emits `None` until its input has a value.
-pub struct Identity {
+/// One-input scalar gain: emits `input * factor`. Emits `None` until its
+/// input has a value (warm-up filter, C8).
+pub struct Scale {
+    factor: f64,
     out: [Cell; 1],
 }
 
-impl Identity {
-    pub fn new() -> Self {
-        Self { out: [Cell::from_f64(0.0)] }
+impl Scale {
+    pub fn new(factor: f64) -> Self {
+        Self { factor, out: [Cell::from_f64(0.0)] }
     }
     pub fn builder() -> PrimitiveBuilder {
         PrimitiveBuilder::new(
-            "__NS__::Identity",
+            "__NS__::Scale",
             NodeSchema {
                 inputs: vec![PortSpec {
                     kind: ScalarKind::F64,
@@ -138,20 +140,14 @@ impl Identity {
                     name: "value".into(),
                 }],
                 output: vec![FieldSpec { name: "value".into(), kind: ScalarKind::F64 }],
-                params: vec![],
+                params: vec![ParamSpec { name: "factor".into(), kind: ScalarKind::F64 }],
             },
-            |_| Box::new(Identity::new()),
+            |p| Box::new(Scale::new(p[0].f64())),
         )
     }
 }
 
-impl Default for Identity {
-    fn default() -> Self {
-        Self::new()
-    }
-}
-
-impl Node for Identity {
+impl Node for Scale {
     fn lookbacks(&self) -> Vec<usize> {
         vec![1]
     }
@@ -160,23 +156,23 @@ impl Node for Identity {
         if w.is_empty() {
             return None;
         }
-        self.out[0] = Cell::from_f64(w[0]);
+        self.out[0] = Cell::from_f64(w[0] * self.factor);
         Some(&self.out)
     }
     fn label(&self) -> String {
-        "__NS__::Identity".to_string()
+        format!("__NS__::Scale({})", self.factor)
     }
 }
 
 fn vocabulary(type_id: &str) -> Option<PrimitiveBuilder> {
     match type_id {
-        "__NS__::Identity" => Some(Identity::builder()),
+        "__NS__::Scale" => Some(Scale::builder()),
         _ => None,
     }
 }
 
 fn type_ids() -> &'static [&'static str] {
-    &["__NS__::Identity"]
+    &["__NS__::Scale"]
 }
 
 aura_core::aura_project! {
@@ -186,7 +182,7 @@ aura_core::aura_project! {
 }
 "#;
 
-const SIGNAL_JSON: &str = r#"{"format_version":1,"blueprint":{"name":"__NAME_SNAKE___signal","nodes":[{"primitive":{"type":"SMA","name":"fast","bound":[{"pos":0,"name":"length","kind":"I64","value":{"I64":2}}]}},{"primitive":{"type":"SMA","name":"slow","bound":[{"pos":0,"name":"length","kind":"I64","value":{"I64":4}}]}},{"primitive":{"type":"Sub"}},{"primitive":{"type":"Bias","name":"bias","bound":[{"pos":0,"name":"scale","kind":"F64","value":{"F64":0.5}}]}},{"primitive":{"type":"__NS__::Identity"}}],"edges":[{"from":0,"to":2,"slot":0,"from_field":0},{"from":1,"to":2,"slot":1,"from_field":0},{"from":2,"to":3,"slot":0,"from_field":0},{"from":3,"to":4,"slot":0,"from_field":0}],"input_roles":[{"name":"price","targets":[{"node":0,"slot":0},{"node":1,"slot":0}],"source":"F64"}],"output":[{"node":4,"field":0,"name":"bias"}]}}"#;
+const SIGNAL_JSON: &str = r#"{"format_version":1,"blueprint":{"name":"__NAME_SNAKE___signal","nodes":[{"primitive":{"type":"SMA","name":"fast","bound":[{"pos":0,"name":"length","kind":"I64","value":{"I64":2}}]}},{"primitive":{"type":"SMA","name":"slow","bound":[{"pos":0,"name":"length","kind":"I64","value":{"I64":4}}]}},{"primitive":{"type":"Sub"}},{"primitive":{"type":"Bias","name":"bias","bound":[{"pos":0,"name":"scale","kind":"F64","value":{"F64":0.5}}]}},{"primitive":{"type":"__NS__::Scale","bound":[{"pos":0,"name":"factor","kind":"F64","value":{"F64":1.0}}]}}],"edges":[{"from":0,"to":2,"slot":0,"from_field":0},{"from":1,"to":2,"slot":1,"from_field":0},{"from":2,"to":3,"slot":0,"from_field":0},{"from":3,"to":4,"slot":0,"from_field":0}],"input_roles":[{"name":"price","targets":[{"node":0,"slot":0},{"node":1,"slot":0}],"source":"F64"}],"output":[{"node":4,"field":0,"name":"bias"}]}}"#;
 
 const CLAUDE_MD: &str = r#"# __NAME__ — an aura research project
 
@@ -311,10 +307,10 @@ mod tests {
             assert!(!out.contains("__"), "unsubstituted token in: {out}");
         }
         let lib = render(LIB_RS, &spec);
-        assert!(lib.contains("\"demo_lab::Identity\""));
+        assert!(lib.contains("\"demo_lab::Scale\""));
         assert!(lib.contains("namespace: \"demo_lab\""));
         let json = render(SIGNAL_JSON, &spec);
-        assert!(json.contains("\"type\":\"demo_lab::Identity\""));
+        assert!(json.contains("\"type\":\"demo_lab::Scale\""));
         assert!(json.contains("\"name\":\"demo_lab_signal\""));
         let cargo = render(CARGO_TOML, &spec);
         assert!(cargo.contains("name = \"demo-lab\""));

crates/aura-cli/tests/project_new.rs

@@ -64,7 +64,7 @@ fn new_scaffold_builds_and_runs_the_authoring_loop() {
     let introspect = aura(&["graph", "introspect", "--vocabulary"], &proj);
     assert!(introspect.status.success());
     let text = String::from_utf8_lossy(&introspect.stdout);
-    assert!(text.contains("demo_lab::Identity"), "project id listed");
+    assert!(text.contains("demo_lab::Scale"), "project id listed");
     let _ = std::fs::remove_dir_all(&base);
 }
 
@@ -136,8 +136,8 @@ fn new_namespace_override_reaches_the_built_vocabulary() {
     let introspect = aura(&["graph", "introspect", "--vocabulary"], &proj);
     assert!(introspect.status.success());
     let text = String::from_utf8_lossy(&introspect.stdout);
-    assert!(text.contains("custom_ns::Identity"), "overridden namespace listed: {text}");
-    assert!(!text.contains("demo_lab::Identity"), "default-derived namespace must not leak");
+    assert!(text.contains("custom_ns::Scale"), "overridden namespace listed: {text}");
+    assert!(!text.contains("demo_lab::Scale"), "default-derived namespace must not leak");
     let _ = std::fs::remove_dir_all(&base);
 }