feat(formplayer): Enable AJV $data cross-field validation & computed fields engine

[najuna-brian]

Description

Adds two core capabilities to the formplayer and formulus extension pipeline:
Closes #304

1. $data cross-field validation: AJV now supports $data references, enabling schema rules like {"const": {"$data": "1/otherField"}} to validate that one field matches another.
2. Computed fields engine: Apps can declare auto-calculated fields in ext.json. The formplayer watches dependencies, calls the registered extension function, and updates the field value automatically.

Why

• QR code validation: Forms used at different points in a project may require the QR code scanned at the end to match the one assigned at the start. $data rules enforce this in the schema with a clear error message no custom code needed per form.
• Auto-generated PDC codes : Household codes like 2-2-4-A (region-sector-ponto-household) must be computed from multiple fields. The computed fields engine lets apps define this logic in ext.json + a helper JS module, keeping the formplayer generic.

Changes

formulus-formplayer (6 files):

• App.tsx : Enable $data: true in AJV; expose window.formulusCurrentFormData; add computed fields processing in handleDataChange
• ExtensionsLoader.ts : Add computedFields to metadata and load result interfaces
• FormulusInterfaceDefinition.ts : Add computedFields to ExtensionMetadata
• FormEvaluationContext.tsx : Export shared ExtensionFunction type
• builtinExtensions.ts : Use shared type, fix lint issues
• DynamicEnumControl.tsx : Fix lint warnings, apply prettier

formulus (2 files):

• ExtensionService.ts : Add computedFields to interfaces; pass through from ext.json; deep merge by formType; fix pre-existing type errors
• FormplayerModal.tsx : Include computedFields in the extensions object sent to the formplayer WebView

How apps use it

$data validation in schema.json:

{ "const": { "$data": "1/qr_captura" } }

Computed fields in ext.json:

"computedFields": {
  "pdc": {
    "pdc_name": {
      "function": "computePdcName",
      "dependencies": ["region", "sector", "ponto", "ponto_colheita", "codigo_amostra"]
    }
  }
}

The function is a standard extension function loaded from a JS module. It receives the full form data and returns the computed value.