fix: Linked table nested form lifecycle - preserve parent form state across child form sessions

[najuna-brian]

Description

Summary

Linked-table question types in AnthroCollect need to open a child form on top of a parent form, wait for the child to be completed or dismissed, and then return to the parent form exactly where it was — same page, same field state, same scroll position. This matches the behavior that existed in the original OMO repo running on top of ODK-X.

This PR fixes the root cause: the native Formulus app was destroying the parent form modal before opening the child form, making state restoration impossible.

---

Problem

The original App.tsx maintained a single formplayer modal instance. When openFormplayer was called while a modal was already visible, it explicitly closed the existing modal first:

// Old code — parent is torn down before child opens
if (formplayerVisibleRef.current) {
  setFormplayerVisible(false);
  await new Promise<void>(resolve => setTimeout(() => resolve(), 300));
}

This meant:

• The parent form's React state was unmounted and lost
• The parent's current page position was lost
• The parent's WebView was destroyed and would have to reinitialise from scratch
• Returning to the parent after the child completed required the parent to reload entirely

---

Solution

Replace the single-modal design with a modal stack. Each openFormplayer call pushes a new entry onto the stack. The parent modal stays mounted underneath the child modal. When the child closes, the parent becomes the top of the stack again and receives a focus notification so the form JS layer can react.

Key concepts

Concept	Detail
formplayerStack	Array of FormplayerStackEntry objects, one per open form level
isActive prop	Only the top-of-stack modal is isActive=true; all others are false
Submission guard	Submission handler only registers when visible && isActive - inactive parent modals don't compete
State preservation	Form state reset only fires when visible goes false, not when isActive changes
Focus handoff	When isActive transitions false → true on a ready WebView, onReceiveFocus() is injected into the WebView JS context
Retry initialiser	initializeStackEntry polls for the modal ref up to 20 times × 100ms to handle slow mounts before calling initializeForm

[r0ssing]

Let's rename to sub-observation as agreed

[najuna-brian]

Let's rename to sub-observation as agreed

Thanks @r0ssing,
This will be addressed in a follow-up PR as agreed.