Add CQL execution library for SDC and clinical reasoning

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Summary

HAIstack SDC currently models cqf-library metadata and defines a CQLProvider / CQLExpressions adapter in pkg/sdc, but there is no CQL runtime. Questionnaires that depend on CQL expressions cannot be evaluated today.

Problem

• SDC supports CQL in expressions (language: application/x-cql) per the SDC IG
• pkg/sdc only evaluates FHIRPath by default (FHIRPathExpressions, NewSDCFHIRPathEngine)
• CQLExpressions fails with an unavailable-expression diagnostic when no provider is installed
• cqf-library extensions are round-tripped as metadata only

Proposed scope

Create a dedicated library (suggested: pkg/cql or separate module) that:

1. Loads CQL libraries (from canonical references / contained resources / terminology store)
2. Evaluates CQL expressions against a context (subject, launch context, questionnaire variables)
3. Integrates with SDC via the existing CQLProvider interface
4. Documents boundaries vs FHIRPath (when to use each)

Integration points

• pkg/sdc: CQLProvider, CQLExpressions, population/validation/render expression providers
• pkg/runtime / pkg/http: wire default provider when libraries are available
• modules/sdc: example questionnaires using CQL

Out of scope (initial)

• Full CQF Measure evaluation
• Clinical quality reporting pipelines

Acceptance criteria

• CQL library can evaluate a simple expression against a Patient context
• SDC $populate / $validate / Render can use CQL when CQLProvider is wired
• Clear error when library or context is missing
• Tests and README for adapter wiring

Related

• SDC full compliance PR: Full SDC compliance: expression context, HTTP wiring, extensions #25
• pkg/sdc/types.go (CQLProvider, CQLExpressions)
• pkg/sdc/README.md (adapter boundaries)