fix: preserve unresolved FCD DataSet members for signal selection

[masarray]

Problem

Physical Siemens SIPROTEC IEDs can expose DataSet members as FCD/DataObject-level entries (daName=""). The DataSet directory is discovered correctly, but members without a uniquely resolved primary DataAttribute can disappear before application signal selection.

Fix

• Add an engine-owned Iec61850DataSetSignalInventoryProjection.
• Prefer the existing typed/resolved mandatory primary signal descriptors.
• Preserve every remaining static DataSet member as an unresolved mandatory descriptor instead of silently dropping it.
• Keep DataSet identity, member index, FC, MMS reference, CDC/context where available, and RCB membership evidence.
• Do not invent a leaf DataAttribute when the engine cannot prove one.

Regression coverage

• Synthetic Siemens-like case with 2 DataSets / 58 FCD-only members (36 digital + 22 analog) must return 58/58 mandatory selection rows.
• Existing resolved stVal primary target wins and does not create a duplicate FCD placeholder.

This intentionally separates DataSet member inventory completeness from later readable-primary-value resolution.

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