Fix IndexOutOfBoundsException in MultiSelectionLayout when a new seletion starts past every selectable

[Bitsy (bitsycore)]

MultiSelectionLayout.crossStatus assumed that when startSlot == endSlot, at least one of them was the non-dragging handle anchored ON a text, so both slots had to be odd and slot / 2 was a valid info index

That assumpton fails when a fresh mouse-down starts a NEEW selection and the gesture position lands past every selectable in the SelectionContainer so the SelectionLayoutBuilder.updateSlot returns AFTER for every appended info, so both startSlot and endSlot stay UNASSIGNED_SLOT and get defaulted to lastSlot = 2 * infoList.size inside of build()

Now both slots are equal and even, startSlot / 2 == infoList.size is out of bounds, and every accessor that reads infoList through slotToIndex throws IndexOutOfBoundsException

Testing

Reproduce with, on jvm desktop for ex with

    SelectionContainer(Modifier.fillMaxSize()) {
        Column {
            repeat(3) { BasicText("chunk ${it + 1}") }
            Spacer(Modifier.height(500.dp))
        }
    }

Mouse-press in the empty area below "chunk 3", it would throws IndexOutOfBoundsException: Index 3 out of bounds for length 3 at MultiSelectionLayout.getCrossStatus(SelectionLayout.kt:184).