Implement incremental Ignixa SDK adoption and FHIRPath seam

[Brendan Kowitz (brendankowitz)]

Summary

• establish one high-level architectural decision for incrementally adopting the Ignixa SDK through narrow, independently selectable capability seams
• introduce a provider seam for FHIRPath evaluation with Firely as the default and Ignixa 0.6.73 as an opt-in implementation
• migrate evaluating call sites and search indexing while preserving Firely-backed FHIRPath Patch node selection
• add cross-version characterization, generated-expression, search-index, resolver, and failure-metric parity coverage

Architecture

The migration is governed by a single ADR: docs/arch/adr-2608-ignixa-sdk-migration.md.

The decision is to migrate capability by capability, retain Firely as the default for unmigrated or unapproved behavior, avoid silent runtime fallback, and require semantic parity plus production-shaped performance evidence before enabling an Ignixa seam.

Configuration

"FhirSdkProvider": {
  "Default": "Firely",
  "Import": "Firely",
  "FhirPath": "Ignixa"
}

Import and FhirPath remain independently selectable so each capability can be evaluated and rolled back separately.

Validation

• targeted FHIRPath/indexer tests across STU3, R4, R4B, and R5
• API module composition tests across all supported versions
• R4 resource-parser build
• generated and composite search-parameter expression parity
• non-vacuous resource and search-index parity corpus
• mixed-model reviews using Claude Opus, GPT Sol, and Gemini

Review notes

• Firely remains the default provider
• indexing and reindexing share the same selected provider
• $patch node selection remains Firely-backed until its element-model behavior can migrate without losing node identity
• ABTestRunner performance-workload changes were removed from this pull request

[Codecov Comments Bot (codecov-commenter)]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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