Add StructureMap runtime for SDC extraction

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Summary

SDC questionnaires can declare sourceStructureMap for Tier-5 extraction. HAIstack defines StructureMapExtractor in pkg/sdc but does not ship a StructureMap execution engine. Extraction fails when a questionnaire references a map and no runtime is plugged in.

Problem

• QuestionnaireExtractor routes to StructureMapExtractor when sourceStructureMap is set
• Default runtime wires QuestionnaireExtractor without StructureMap.Run
• Error today: StructureMap runtime is unavailable for <canonical>
• Definition/template extraction covers many cases, but map-based extraction is a normative SDC path

Proposed scope

Implement a StructureMap runtime (suggested: pkg/structuremap or separate module) that:

1. Loads StructureMap resources (store / registry / canonical resolution)
2. Executes map groups/rules against Questionnaire + QuestionnaireResponse inputs
3. Produces FHIR resources for transaction bundle entries
4. Integrates via existing StructureMapExtractor.Run hook

Integration points

• pkg/sdc/assembly.go — StructureMapExtractor
• pkg/sdc/extraction.go — QuestionnaireExtractor.StructureMap
• pkg/runtime/wire.go — optional default wiring when maps are present
• modules/sdc/examples/extraction-template.json — example map artifact

Out of scope (initial)

• Full FHIR R4/R5 StructureMap transform catalog parity on day one
• CQL transforms inside maps (depends on CQL library issue)

Acceptance criteria

• Can execute a minimal StructureMap producing one resource from a QuestionnaireResponse
• QuestionnaireExtractor uses runtime when sourceStructureMap is configured
• Clear diagnostics when map is missing or execution fails
• Tests mirroring TestQuestionnaireExtractorUsesMetadata with a real engine

Related

• SDC full compliance PR: Full SDC compliance: expression context, HTTP wiring, extensions #25
• pkg/sdc/assembly.go (StructureMapExtractor)
• pkg/sdc/gaps_test.go (stub Run example)