fix(engine): labeled alternation with definition reference

[zharinov]

Summary

• Fix bug where labeled alternation variants referencing named definitions with captures produced empty $data: {}
• The type system already correctly inferred $data: { name: Node }, but runtime materialization discarded the value

Problem

When a labeled alternation variant references a named definition containing captures:

Inner = (identifier) @name
Plugin = [A: (Inner)]

The $data field was empty {} instead of containing { name: Node }.

Root Cause

In materializer.rs, EndEnum was discarding any pending value from an inner definition's Obj/EndObj and constructing $data from the empty fields vector.

Fix

Modified EndEnum to consume the pending value if available:

let data = pending.take().unwrap_or_else(|| Value::Object(fields));

Test plan

• Added regression test alternations_tagged_with_definition_ref
• All 751 tests pass
• Verified fix with manual testing