Adds the valence-works/bpmn package feed and pins Bpmn.Model,
Bpmn.Semantics and Bpmn.Interchange at 0.1.1-preview.19, then wires
two new module projects consuming those libraries without
reimplementing anything they provide (D12): Elsa.Bpmn for BPMN
execution and Elsa.Bpmn.Interchange for XML import/export, kept as a
separate package so hosts that only execute BPMN don't take the XML
reader. Both are marked IsPackable=false until the Bpmn.* packages
are published to nuget.org. Test projects are added for both, unit
and integration, and all four are wired into Elsa.sln so PR CI
discovers and runs them. This unblocks #7925 and the rest of the
BPMN runtime work in #7909.
Co-authored-by: Claude Opus 5 <noreply@anthropic.com>
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