Fix live selected-RCB DataSet export

[masarray]

Problem

Some IEDs report a populated DataSet during the explicit RCB availability check, while the earlier broad discovery model contains only MemberCount and an empty member-reference list. ARSAS therefore showed 1 FCDA but live selected-RCB export failed with references an empty DataSet.

Fix

• preserve the exact GetDataSetDirectory members and deletable flag in MmsRcbAvailabilitySnapshot;
• add LiveRcbDataSetEvidenceMerger to merge the selected RCB's verified DataSet directory into a cloned live model;
• never mutate the original discovery model;
• reject export with a clear instruction when Check Availability evidence is absent, unreadable, or contains no usable references;
• retain the existing selected-RCB filter and SCL exporter after the model is enriched.

Regression coverage

The test reproduces the field case directly: discovery says MemberCount=1 but Members=[]; availability returns one exact member. The merged model then passes selected-RCB filtering while the original model remains unchanged.

Validation

Public-source checks, Release build, and all automated tests passed. Temporary implementation workflows and patch files were removed before review.

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