Add captured self specs for scope across render boundaries

[karreiro]

Adds two new conformance specs in specs/liquid_ruby/bare_bracket_self.yml that define how a captured self behaves when crossing {% render %} boundaries. The first spec shows that self captured in the outer template continues to resolve through the original scope both before and inside the rendered snippet, while a freshly read self inside the snippet resolves to the snippet's own scope (outer|outer|snippet). The second spec extends this across three nested renders, proving that each level's captured self remains stable as render depth increases, while a fresh self in the innermost snippet sees its own scope (1|1/2|1/2/3).

These specs belong in liquid-spec because this behavior is a cross-implementation contract, not an implementation detail. They pass on both the Ruby reference adapter and the liquid-vm adapter, replacing prior liquid-vm-only integration tests. The corresponding liquid-vm PR updates the spec reference and removes the duplicated tests in favor of these shared conformance specs.