fix(zig): address fifth gitnexus-check review pass

- tree-sitter-languages test: the Zig "functions, structs, enums, and
  imports" case now asserts the `import.source` capture for
  `const std = @import("std");` (the fixture's only import), so a query
  change that drops Zig import matching fails it instead of passing
  unchanged.

Not re-fixed:
- "Return type is read from the wrong tree-sitter field": carried over from
  the fourth pass unchanged. tree-sitter-zig 1.1.2 has no `return_type`
  field on function_declaration; the return type IS the `type` field, and
  the pin test added in the fourth-pass commit
  (zig-extractors.test.ts, `childForFieldName('return_type')` is null,
  `returnType` = `void` / `!*Counter`) proves it.
- "`DIR_LANG` has no zig entry": carried over unchanged for the third time;
  the entry exists since the second-pass commit.

gitnexus/test/integration/tree-sitter-languages.test.ts

@@ -712,6 +712,17 @@ describe('Tree-sitter multi-language parsing', () => {
       expect(defTypes).toContain('definition.function');
       expect(defTypes).toContain('definition.struct');
       expect(defTypes).toContain('definition.enum');
+
+      // `const std = @import("std");` must yield an import.source capture —
+      // without this assertion a query change that drops Zig import matching
+      // would pass this test unchanged.
+      const imports: string[] = [];
+      for (const match of matches) {
+        for (const capture of match.captures) {
+          if (capture.name === 'import.source') imports.push(capture.node.text);
+        }
+      }
+      expect(imports).toEqual(['"std"']);
     });
 
     it('reports Zig unavailable and throws "Unsupported language" when the grammar is absent', async () => {