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>

codegraph-kernel/src/rustlang.rs

@@ -446,7 +446,9 @@ impl<'t> Walker<'t> {
         } else if kind == "trait_item" {
             self.extract_interface(node);
             skip_children = true;
-        } else if kind == "struct_item" {
+        } else if matches!(kind, "struct_item" | "union_item") {
+            // `union_item` mirrors structTypes on the TS side: same `body:`
+            // field, same extractor, kind "struct" (NodeKind has no "union").
             self.extract_struct(node);
             skip_children = true;
         } else if kind == "enum_item" {
@@ -1130,7 +1132,7 @@ impl<'t> Walker<'t> {
         }
 
         // Structural nodes inside bodies.
-        if kind == "struct_item" {
+        if matches!(kind, "struct_item" | "union_item") {
             self.extract_struct(node);
             return;
         }