fix(c,cpp,objc,rust): index union declarations

A `union` declaration produced no symbol at all in any of the four
languages that have one. The type never entered the graph, and neither
did anything attached to it — in Rust every `impl Trait for MyUnion`
lost its edge, and the impl's methods were left with a qualifiedName
pointing at a type the graph did not contain.

`union_specifier` / `union_item` were absent from the extraction layer
entirely: no `<x>Types` list on the TS side, no dispatch branch in
either kernel walker.

They join `structTypes` (kind `struct` — NodeKind has no `union`), which
is the extension point the table-driven extractors already provide. The
body guard in extractStruct is untouched, so a bodiless `union U;` stays
a forward declaration and is still skipped, exactly like `struct U;`.

`resolveTypeAliasKind` accepts `union_specifier` too, so
`typedef union { … } N;` takes the typedef's name the way
`typedef struct { … } N;` already did. Without it the anonymous union
body would mint a second `<anonymous>` node beside the alias.

Both walkers change together so kernel<->wasm parity holds.

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

src/extraction/languages/objc.ts

@@ -102,7 +102,9 @@ export const objcExtractor: LanguageExtractor = {
   methodTypes: ['method_definition'],
   interfaceTypes: ['protocol_declaration'],
   interfaceKind: 'protocol',
-  structTypes: ['struct_specifier'],
+  // Objective-C is a C superset: `union U { … };` is a definition, same as in
+  // the C extractor.
+  structTypes: ['struct_specifier', 'union_specifier'],
   enumTypes: ['enum_specifier'],
   enumMemberTypes: ['enumerator'],
   typeAliasTypes: ['type_definition'],
@@ -128,7 +130,12 @@ export const objcExtractor: LanguageExtractor = {
       const child = node.namedChild(i);
       if (!child) continue;
       if (child.type === 'enum_specifier' && getChildByField(child, 'body')) return 'enum';
-      if (child.type === 'struct_specifier' && getChildByField(child, 'body')) return 'struct';
+      // `typedef union { … } name;` resolves like `typedef struct` — see the C extractor.
+      if (
+        (child.type === 'struct_specifier' || child.type === 'union_specifier') &&
+        getChildByField(child, 'body')
+      )
+        return 'struct';
     }
     return undefined;
   },