feat(cli): three-way unresolved-namespace hint (#241 T3)

A project-namespaced (::) unknown type id now hints by tier: outside
any project the existing "no Aura.toml was found" text stays
byte-identical; inside a data-only project the hint names the missing
node crate and the attach verb (aura nodes new); a loaded node crate
gets no hint (the vocabulary error carries the message).

refs #241

crates/aura-cli/src/graph_construct.rs

@@ -376,18 +376,25 @@ fn gang_fault_prose(e: &CompileError) -> String {
     }
 }
 
-/// (#185) When an unresolved type id LOOKS project-namespaced (`::`) but no
-/// project was loaded for this invocation (`env.provenance()` is `None`),
-/// append a hint pointing at the missing `Aura.toml` — the likeliest cause is
-/// "ran outside the project directory", not "genuinely unknown std id". Bare
-/// std-vocabulary ids (no `::`) and invocations that DO have a project loaded
-/// get no hint.
+/// (#185/#241) When an unresolved type id LOOKS project-namespaced (`::`),
+/// the likeliest causes are environmental, tiered: outside any project the
+/// hint points at the missing `Aura.toml`; inside a data-only project it
+/// points at the missing node crate. Bare std ids (no `::`) and invocations
+/// with a loaded node crate get no hint (the vocabulary error carries the
+/// message).
 pub(crate) fn unresolved_namespace_hint(e: &LoadError, env: &crate::project::Env) -> Option<String> {
     match e {
-        LoadError::UnknownNodeType(t) if t.contains("::") && env.provenance().is_none() => Some(
-            "type id looks project-namespaced but no Aura.toml was found — run inside a project directory"
-                .to_string(),
-        ),
+        LoadError::UnknownNodeType(t) if t.contains("::") => match env.provenance() {
+            None => Some(
+                "type id looks project-namespaced but no Aura.toml was found — run inside a project directory"
+                    .to_string(),
+            ),
+            Some(p) if p.namespace.is_none() => Some(
+                "type id looks project-namespaced but this project binds no node crate — attach one with `aura nodes new <name>`"
+                    .to_string(),
+            ),
+            Some(_) => None,
+        },
         _ => None,
     }
 }