Add guarded RCB availability and selected-RCB SCL export

[masarray]

Summary

Adds engine-owned, read-only RCB availability inspection and object-model selected-RCB SCL export for legacy SAS workflows.

RCB availability

• bounded batch availability check over discovered Report Control Blocks;
• refreshes DatSet, RptID, ConfRev, RptEna, Resv / ResvTms and best-effort Owner without writing or reserving an RCB;
• reads each distinct referenced DataSet directory once and reports exact FCDA member counts;
• distinguishes Available, InUse, UsedByCaller, Unknown, missing DataSet, empty DataSet, and unreadable DataSet;
• never converts missing Edition 1 reservation evidence into a false green Available state;
• lets the caller identify RCBs owned by its own active report sessions.

SCL selected-RCB export

• inventories ReportControl elements with stable selection keys, type, DataSet binding, member count, indexed state, and instance count;
• clones the source document and never mutates the original;
• retains exactly one selected ReportControl and removes every unselected RCB;
• validates that the retained RCB resolves a populated DataSet in the same Logical Node;
• collapses indexed RptEnabled max to one instance and can rename the exported block to the exact runtime RCB name;
• preserves all DataSets by default; optional cleanup removes only DataSets not referenced by remaining Report/GOOSE/SV/Log controls;
• supports filtering a typed live-discovery model to one RCB.

Legacy SAS CID exporter

• normalizes vendor SCL through the existing interoperable converter;
• applies the selected-RCB filter;
• applies Edition 1 V1.6 or Edition 2 V3.1 compatibility fields;
• writes the CID plus JSON and Markdown evidence;
• reparses and validates that exactly one RCB and a populated DataSet remain.

Validation

• source, provenance, wording, and license boundary checks passed;
• Release build passed;
• complete automated test suite passed;
• new regression tests cover availability truthfulness, caller ownership, occupied/empty/unreadable DataSets, source immutability, indexed RCB collapse, one-RCB retention, Edition 1 profile output, and typed live-model filtering.

Safety boundary

The availability workflow is read-only. It does not write RptEna, Resv, ResvTms, Owner, or DatSet, and it does not claim conformance or universal vendor interoperability.

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