enhancement: FHIRIsModifier has no active enforcement — document or implement modifier element impact checking

[Ardenexal]

Component

Validation

FHIR Version

R4, R4B, R5 (all versions)

Problem Statement

FHIRIsModifier is a PHP attribute with no associated Symfony validator and no enforcement in FHIRValidationService. The FHIR conformance spec states implementations "SHALL understand the impact of modifier elements." Our validator marks modifier properties but never checks whether the consuming application can handle them.

Proposed Solution

Three options:

1. Active validation: For each #[FHIRIsModifier] property, check whether the value changes resource semantics (e.g. status = entered-in-error). Complex — requires per-property lists.
2. Informational violation (middle ground): When a modifier element has a non-default/meaning-changing value, emit INFO noting the modifier is present and must be understood by the consumer.
3. Documentation only (minimum): Document in README and compatibility matrix that modifier element impact checking is the application layer responsibility.

Additional Context

validateModifierExtensions() already handles unknown modifier extensions. This issue covers known modifier elements (typed properties marked isModifier=true).