test(engine): add regression test for enum tag with definition refs

[zharinov]

Summary

Adds regression test for BUG #6: Tagged alternation used wrong $tag when branches reference named definitions.

The fix is already in master. This PR adds the regression test to prevent future regressions.

The Bug

When a definition name (e.g., Second) is interned before a branch label (e.g., First), the enum member indices would mismatch because:

• Enum members are emitted in BTreeMap (Symbol interning) order
• But the compiler was using AST branch order for member indices

Test Case

Second = (binary_expression left: (number) @left right: (number) @right)
Q = (program (expression_statement [First: (number) @num  Second: (Second) @bin]))

With input 42, the output should be {"$tag": "First", ...} (not "Second").