feat(iOS, Tabs): partially implement RFC-1028 for Tabs on iOS (#3781)

## Description

Partially implements
[RFC-1028](#3702)
for the Tabs component on iOS. This mirrors what was already done for
Android in #3776, adapting the iOS native implementation to the new
communication model.

Two things are explicitly out of scope for this PR and will be addressed
in follow-up PRs:
- **State conflict resolution** logic (as described in RFC-1028)
- **`moreNavigationController` support** — it is currently handicapped
and will be brought up to speed separately

Related to #3702.

## Changes

### iOS (native)

- **Introduced `RNSTabsNavigationState`**: A new struct/class that
follows the RFC-1028 model — `selectedScreenKey` + `provenance`. It
carries the information about which tab is selected and whether the
change originated from the native side or from JS.

- **Refactored container update logic**: JS-initiated and
native-initiated tab changes now have separate entry paths that converge
into a common set of helper methods to update the state. Native
navigation is slightly special — `UITabBarController` manages
`selectedViewController` internally, so the state update path for
native-initiated changes differs slightly from the JS-initiated path.

- **Moved `UITabBarControllerDelegate` methods into
`RNSTabBarController`**: Previously handled by an external
`RNSTabBarControllerDelegate`. Since the controller needs to update
navigation state on user actions (which only the delegate is aware of),
inlining the delegate into the controller is a cleaner fit. The external
`RNSTabBarControllerDelegate` class has been removed entirely.

- **Removed `RNSTabBarControllerSelectionObserving` protocol**: No
longer needed after the delegate consolidation.

- **Replaced `OnNativeFocusChange` with `OnTabChange` event**: The new
event follows RFC-1028 and carries the extended `RNSTabsNavigationState`
payload. Aligns with the Android implementation from #3776.

- **`TabsScreen` no longer exposes `isFocused`**: Screen-level selection
awareness has been removed on iOS, consistent with the Android changes.

- **Renamed `needsUpdateOfReactChildrenControllers` →
`needsUpdateOfChildViewControllers`**: Cleanup rename for accuracy.

### JS / TypeScript

- **Updated `TabsHost.ios.tsx`**: Wired up the new `navState` prop and
`onTabChange` event, aligning iOS JS bindings with the updated native
interface and with the Android implementation.
- **Updated `TabsHostIOSNativeComponent.ts`**: Reflects the new native
props and events (`navState`, `onTabChange`, removal of
`onNativeFocusChange`).
- **Removed `isFocused` from `TabsScreen.ios.tsx`**: Consistent with the
screen no longer managing its own selection state.

## Test plan

- Tested manually using the existing tabs example app on iOS.
- Verified basic tab navigation works for both native-initiated (user
tap) and JS-initiated tab changes.

## Checklist

- [x] Included code example that can be used to test this change.
- [ ] Updated / created local changelog entries in relevant test files.
- [ ] For visual changes, included screenshots / GIFs / recordings
documenting the change.
- [x] For API changes, updated relevant public types.
- [ ] Ensured that CI passes

Author: kkafar

ios/tabs/host/RNSTabBarController.h

@@ -2,6 +2,7 @@
 
 #import <UIKit/UIKit.h>
 #import "RNSTabBarAppearanceCoordinator.h"
+#import "RNSTabsNavigationState.h"
 #import "RNSTabsScreenViewController.h"
 
 #if !TARGET_OS_TV
@@ -41,6 +42,7 @@ NS_ASSUME_NONNULL_BEGIN
  * Get reference to the host component view that owns this tab bar controller.
  *
  * Might return null in cases where the controller view hierararchy is not attached to parent.
+ * The reference is retained strongly and it is expected to be managed by `TabsHost`.
  */
 @property (nonatomic, readonly, nullable) RNSTabsHostComponentView *tabsHostComponentView;
 
@@ -50,14 +52,24 @@ NS_ASSUME_NONNULL_BEGIN
  */
 @property (nonatomic, readonly, strong, nonnull) RNSTabBarAppearanceCoordinator *tabBarAppearanceCoordinator;
 
+/**
+ * Represents current navigation state.
+ *
+ * After each model update, the container (controller) updates the navigation state. The `provenance` part is
+ * incremented monotonically with each state update.
+ *
+ * The controller manages this state. It MUST NOT be overwritten by any external actor.
+ */
+@property (nonatomic, readonly, strong, nullable) RNSTabsNavigationState *navigationState;
+
 /**
  * Update tab controller state with previously provided children.
  *
  * This method does nothing if the children have not been changed / update has not been requested before.
  * The requested update is performed immediately. If you do not need this, consider just raising an appropriate
  * invalidation signal & let the controller decide when to flush the updates.
  */
-- (void)updateReactChildrenControllersIfNeeded;
+- (void)updateChildViewControllersIfNeeded;
 
 /**
  * Force update of the tab controller state with previously provided children.
@@ -68,7 +80,7 @@ NS_ASSUME_NONNULL_BEGIN
 - (void)updateReactChildrenControllers;
 
 /**
- * Find out which tab bar controller is currently focused & select it.
+ * If any state update operation is pending - perform it.
  *
  * This method does nothing if the update has not been previously requested.
  * If needed, the requested update is performed immediately. If you do not need this, consider just raising an
@@ -77,7 +89,9 @@ NS_ASSUME_NONNULL_BEGIN
 - (void)updateSelectedViewControllerIfNeeded;
 
 /**
- * Find out which tab bar controller is currently focused & select it.
+ * Update the selected view controller to what's currently requested.
+ *
+ * To request update use `setPendingNavigationStateUpdate`.
  *
  * The requested update is performed immediately. If you do not need this, consider just raising an appropriate
  * invalidation signal & let the controller decide when to flush the updates.
@@ -161,18 +175,20 @@ NS_ASSUME_NONNULL_BEGIN
 - (void)childViewControllersHaveChangedTo:(nonnull NSArray<RNSTabsScreenViewController *> *)childViewControllers;
 
 /**
- * Tell the controller that react provided tabs have changed (count / instances) & the child view controllers need to be
- * updated.
+ * Request navigation state update from the controller to the given one.
  *
- * Do not raise this signal only when focused state of the tab has changed - use `needsSelectedTabUpdate` instead.
+ * If you want to execute multiple updates in sequence you must flush the container after each one separately.
  */
-@property (nonatomic, readwrite) bool needsUpdateOfReactChildrenControllers;
+- (void)setPendingNavigationStateUpdate:(nullable RNSTabsNavigationState *)navState;
 
 /**
  * Tell the controller that react provided tabs have changed (count / instances) & the child view controllers need to be
  * updated.
+ *
+ * Do not raise this signal only when you want to modify selected view controller. Use `setPendingNavigationStateUpdate`
+ * instead.
  */
-@property (nonatomic, readwrite) bool needsUpdateOfSelectedTab;
+@property (nonatomic, readwrite) bool needsUpdateOfChildViewControllers;
 
 /**
  * Tell the controller that some configuration regarding the tab bar apperance has changed & the appearance requires