fix: captured tagged alternation in field produces tagged union

[zharinov]

Summary

• Fixed bug where field: [Tag: ...] @cap produced raw Node instead of tagged union {$tag, $data}
• Added unwrap_field_value() helper to look through FieldExpr when detecting enum-producing alternations
• Emit capture effects after EndEnum for correct tagged union capture semantics

Why

Parser intentionally treats field: expr @cap as (field: expr) @cap so quantifiers work on fields (e.g., decorator: (x)*). This meant captures on field values wrapped FieldExpr, and build_capture_effects didn't look through it to detect the enum-producing alternation inside.

Notes

Two complementary changes:

1. capture.rs: unwrap_field_value() + updated creates_structured_scope check
2. expressions.rs: New code path inner_is_scope_creating_scalar for enum capture placement