Serialize signal values consistently

[PauloCarvalho13]

Structured signal values are currently serialized only when a signal is initialized through dataSignal or dataSignals.

val person = dataSignal("person", Person("John Doe", 18))

This correctly renders the initial value as a JavaScript object:

$person = {name: 'John Doe', age: 18}

However, assigning a new Kotlin value through setValue does not use the same serialization logic:

person.setValue(Person("Jane Doe", 21))

The value is currently inserted directly into the generated JavaScript expression rather than being rendered as a JavaScript object literal. This leads to inconsistent behavior between signal initialization and signal updates, and prevents structured values from being safely assigned after initialization.
Serialization of a Kotlin signal value should be owned by the signal API and applied consistently wherever a value is emitted into a Datastar expression.

Possible API

@DatastarSignal
data class Person(
    val name: String,
    val age: Int,
)

val person = dataSignal("person", Person("John Doe", 18))

dataOn(Event.Click) {
    person.setValue(Person("Jane Doe", 21))
}

Expected generated expression:

$person = {name: 'Jane Doe', age: 21}

Assignments from an existing Datastar expression should remain distinct from assignments of Kotlin values:

person.setValue(otherPerson)

Acceptance criteria

• A signal value is rendered consistently during both initialization and setValue.
• Nested typed values are serialized recursively into JavaScript object literals.
• Strings remain JavaScript strings and are not interpreted as raw JavaScript objects.
• Structured values are not rendered through arbitrary toString() output.
• Existing escaping behavior for object keys and string values is preserved.
• Tests cover signal initialization and setValue for scalars, nested values, null values, and escaped strings.

Related to: #15
Related to: #16