fix: nested untagged alternation containing tagged alternation

[zharinov]

Summary

• Fix panic when untagged alternation contains a tagged alternation as a branch (e.g., [[A: (x)]])

Why

The compiler at sequences.rs:227 assumed that if the type is an Enum, all branches must have labels. But a nested untagged alternation like [[A: (x)]] can inherit an enum type from its inner branch while the outer branches have no labels.

The fix checks if branches are syntactically tagged (have labels in AST), not just whether the resulting type is an enum.