Method source_rule_name

The exact CDDL source rule name `ident` was registered under (e.g. `my-rule`, which `RustIdent` camel-cases to `MyRule`, indistinguishable from `my_rule`). `None` for a struct synthesized during IR build (no source rule). The conformance oracle roots its validator here so it targets a PROVABLE spec rule rather than a lossy reversal of the ident.

(&self, ident: &RustIdent)