docs: define policy-aware safety traceability use cases

[devashridatta-dotcom]

Summary

Adds an initial use-case document for policy-aware safety traceability and evidence exchange.

The document covers:

• Tier 1 to OEM to independent-assessment handover;
• internal feature development and responsibility transfer;
• jurisdiction and legal-entity changes;
• OSS component intake and responsibility boundaries;
• incremental evaluation, periodic full evaluation, and tool confidence;
• packaging and provenance boundaries; and
• an end-to-end demonstrator with objective acceptance criteria.

Scope

This is a documentation-only addition to the srac_poc branch. It does not change the SRAC schema, exporter, receiving tools, or any safety decision.

The intent is to provide concrete use cases for the S-CORE traceability-tooling discussion before selecting a canonical metamodel and mapping-overlay architecture.

Related: eclipse-score/score#3282