fix(mir): make BackLink parent removal precise #531
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Describe your changes
BackLink implements PartialEq as always true so that backlink fields are ignored in derived equality and hashing. However,
remove_parent in MIR ops compared BackLink values directly:
Because of the always-true PartialEq, this predicate was always false and effectively cleared the entire parents vector instead of removing only the requested parent.
This change updates all remove_parent implementations for ops that use parents: Vec<BackLink> to compare against the upgraded Link via to_link(), keeping BackLink’s structural Eq/Hash semantics intact while making parent removal semantically correct.