RDF conversion drops top-level redefinition values (attribute :>> x = v), while nested anonymous redefinitions keep them

Found while raising the formal rigor of a reference-architecture model (mission-twin RA, v0.4.2 pinned binary).

Top-level redefinition in a specializing def — value dropped:

part def Block { attribute tempo : Tempo; }
part def Writer :> Block { attribute :>> tempo = Tempo::operative; }

-convert ttl emits Writer with no owned member for the redefined tempo at all: the value is unrecoverable from RDF (evaluation via -e sees it fine).

Nested anonymous redefinition — value kept:

interface def Seam {
    attribute sla : ServiceLevelAgreement {
        attribute :>> committedBy = PartyRole::custodian;  // kept: sysml:redefines "committedBy" + sysml:value
    }
}

The nested form converts as an anonymous AttributeUsage with sysml:redefines + sysml:value — exactly what one would want for the top-level form too.

Workaround we adopted: declare fresh same-named attributes per specializing def (which draws "Duplicate of inherited member name" warnings) or avoid inheritance for valued attributes.

Start with the RDF conversion path used by -convert ttl, then compare its handling of the top-level and nested anonymous redefinition examples in the issue. Use -e and the generated Turtle output to verify the behavior. Done means the top-level redefinition preserves both sysml:redefines and sysml:value, matching the nested form.