Add FormFieldConverter and dataOverlayProvider to Form/FormPanel

[tfonrouge]

Summary

• FormFieldConverter — per-field converter interface enabling custom type transformations between model values and form control values during setData()/getData(), beyond the built-in String/Number/Boolean/Date/KFiles handling
• dataOverlayProvider — optional callback on Form/FormPanel that injects external key-value pairs into getData() results, allowing non-FormControl components (tabulators, custom editors) to participate in form data collection

Problem

KVision's form system handles built-in types during serialization, but custom types require workarounds. Additionally, external components have no way to contribute data to a form without implementing FormControl.

Examples

FormFieldConverter — mapping an enum ordinal to a select control:

val converter = object : FormFieldConverter {
    override fun fromJson(jsonValue: dynamic): Any? =
        Priority.entries[jsonValue as Int].name

    override fun toJson(controlValue: Any?): dynamic =
        Priority.valueOf(controlValue as String).ordinal
}
form.add(MyModel::priority, TomSelect(), converter = converter)

FormFieldConverter — via registerConverter():

form.add(MyModel::score, Text())
form.registerConverter(MyModel::score, myConverter)
// later: form.removeConverter(MyModel::score)

dataOverlayProvider — tabulator contributing row data:

formPanel.dataOverlayProvider = {
    mapOf("rows" to tabulator.getData().serialize())
}
// overlay values bypass validation and take precedence over field values

Design decisions

• FormFieldConverter is a non-generic interface with two methods (fromJson/toJson) — minimal, easy to implement
• converter parameter added to all add()/bind() overloads (20+) in both Form and FormPanel, placed before lambda parameters following Kotlin's lambda-last convention. This is fully compatible with named-argument callers; positional callers will see a compile error prompting them to add the new parameter.
• Both features work in serializer (@Serializable classes) and non-serializer (Map-based) form paths
• Overlay values bypass converter/type dispatch and validation (documented in KDoc). Null overlay values are skipped. Unknown keys cause a deserialization error in serializable forms.
• Converters are cleaned up on remove()/removeAll() and survive clearData()

Test plan

• Converter via string-key add()
• Converter via typed add() (KProperty1 overload)
• registerConverter() / removeConverter() lifecycle
• Converter in non-serializer (Map-based) form path
• Overlay: basic injection, field override, dynamic form
• clearData() preserves converter registrations
• FormPanel-level converter delegation
• getDataJson() with converter

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