Support SDC calculatedExpression in FHIRQuestionnaireValidator

[Ardenexal]

Component: Validation (FHIRQuestionnaireValidator) — not in the template dropdown
FHIR Version: All versions (SDC extension)

Problem Statement

FHIRQuestionnaireValidator does not yet support the SDC calculatedExpression extension
(http://hl7.org/fhir/uv/sdc/StructureDefinition/sdc-questionnaire-calculatedExpression).
Documented as out of scope in docs/validation/questionnaire.md. Answers that should be
auto-calculated from a FHIRPath expression are not checked against the calculated value.

Proposed Solution

Read the calculatedExpression extension off the source Questionnaire item, evaluate it via
FHIRPathService against the response, and validate that the submitted answer matches the
calculated result. Reuse the expression-evaluation pattern from evaluateEnableWhenExpression
(src/Component/Validation/src/FHIRQuestionnaireValidator.php:2254) and the EXT_* constant
convention (~:126-175). Add an EXT_CALCULATED_EXPRESSION_SDC constant.

Additional Context

Precedent: EXT_ENABLE_WHEN_EXPRESSION_SDC (line 175). One of three remaining uncovered
SDC/R5 questionnaire-validation features; enableWhenExpression and regex are already covered.