Align protocol runtime with spec boundaries

Move demo leaf echo behavior out of the core protocol runtime, treat procedure IDs as opaque protocol fields, and return direct registered children in endpoint introspection to match the spec.

src/protocol/validation.rs

@@ -60,7 +60,7 @@ pub fn validate_header(header: &PacketHeader) -> Result<(), ValidationError> {
     Ok(())
 }
 
-/// Validates the canonical dotted `procedure_id` shape.
+/// Validates the protocol-level `procedure_id` invariant.
 pub fn validate_procedure_id(procedure_id: &str) -> Result<(), ValidationError> {
     if procedure_id == INTROSPECTION_PROCEDURE_ID {
         return Ok(());
@@ -72,15 +72,6 @@ pub fn validate_procedure_id(procedure_id: &str) -> Result<(), ValidationError>
         ));
     }
 
-    if !procedure_id
-        .chars()
-        .all(|ch| ch.is_ascii_alphanumeric() || ch == '_' || ch == '.')
-    {
-        return Err(ValidationError::ProcedureId(
-            "procedure identifier should use alphanumeric characters, dots, and underscores",
-        ));
-    }
-
     Ok(())
 }