fix: empty captured sequences produce empty struct instead of null

[zharinov]

Summary

• Fix type inference to recognize truly empty sequences ({ }) as producing empty struct type, not Node
• Fix compiler to emit Obj/EndObj wrapper for empty captured sequences
• Fix materializer to distinguish nested empty objects (captured sequences → {}) from root-level (preamble → null)

Why

Queries like { } @x and { }? @maybe were either panicking with type mismatch or producing incorrect null values. Empty sequences should produce empty struct {} when captured.

Notes

Added is_truly_empty_scope() helper in parser/ast.rs to distinguish truly empty sequences ({ }) from sequences with void flow due to suppression ({(expr) @_}). The latter should still capture the matched node, not produce an empty struct.