docs(union): changelog entry + port-checklist annotations

The two kernel port checklists record the extractor configs as surveyed
at porting time; their structTypes lines are marked superseded rather
than rewritten, so the surveys stay readable as history.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/design/ccpp-kernel-port-checklist.md

@@ -139,7 +139,10 @@ walker mirrors, with file:line anchors (as of `705e501`). Read WITH
 ## Extractor configs (languages/c-cpp.ts — read the whole file when porting)
 
 **cExtractor (line 180):** functionTypes=[function_definition]; NO
-class/method/interface types; structTypes=[struct_specifier];
+class/method/interface types; structTypes=[struct_specifier]
+(superseded: `union_specifier` joined structTypes — a named `union U { … };`
+is a definition, extracted with kind `struct`, and `typedef union { … } N;`
+resolves through resolveTypeAliasKind like `typedef struct`);
 enumTypes=[enum_specifier]; enumMemberTypes=[enumerator];
 typeAliasTypes=[type_definition]; importTypes=[preproc_include];
 callTypes=[call_expression]; variableTypes=[declaration];

docs/design/rust-lang-kernel-port-checklist.md

@@ -54,7 +54,9 @@ Types: functionTypes=[`function_item`, **`function_signature_item`**] (the
 latter = a trait method DECLARATION `fn render(&self);` — extracted so a
 trait's method set is first-class); classTypes=[] (impl blocks instead);
 methodTypes = same two; interfaceTypes=[`trait_item`] with
-**interfaceKind:'trait'**; structTypes=[`struct_item`]; enumTypes=[`enum_item`];
+**interfaceKind:'trait'**; structTypes=[`struct_item`] (superseded:
+`union_item` joined structTypes — same `body` field, same extractor, kind
+`struct`); enumTypes=[`enum_item`];
 enumMemberTypes=[`enum_variant`]; typeAliasTypes=[`type_item`];
 importTypes=[`use_declaration`]; callTypes=[`call_expression`];
 variableTypes=[`let_declaration`, `const_item`, `static_item`].