fix(iOS): support Unicode tab keys in Fabric events

[thiagobrez]

Fixes #560.

PR Description

On Fabric, the tab selection and long-press event payloads passed the Core Foundation kCFStringEncodingUTF8 constant to -[NSString cStringUsingEncoding:], which expects an NSStringEncoding. Non-ASCII tab keys therefore returned NULL, and constructing the generated event's std::string from that pointer aborted the app.

This changes both event paths to build the event key from -[NSString UTF8String], with an empty-string fallback for a nil receiver. That preserves the existing UTF-8 event payload behavior for ASCII keys while safely supporting arbitrary Unicode. The implementation uses Foundation and C++ APIs already available to the current Objective-C++ source, without raising the supported React Native, iOS, or compiler requirements.

The example now includes a Korean tab key, a focused agent-device/Maestro flow covers selection and long press, and a patch changeset is included.

How to test?

• corepack yarn build — passed
• corepack yarn lint — passed (three existing warnings, no errors)
• corepack yarn typecheck — passed
• corepack yarn jest --config '{"preset":"react-native","rootDir":"packages/react-native-bottom-tabs"}' src/__tests__/index.test.tsx --runInBand — passed
• Fabric example iOS build with React Native 0.81.4 — passed
• Focused agent-device/Maestro flow — passed, 1 test / 0 failures in 33.903 seconds (JUnit report, interaction recording)

corepack yarn test still exits non-zero in the unchanged repository-wide harness because @bottom-tabs/react-navigation has no discovered tests. The focused package Jest command above passes; this known harness condition is separate from the branch.

Device verification

Captured with agent-device 0.20.10 against the Fabric example app (bottomtabs.example):

• Before: iPhone 17 Pro simulator, iOS 27.0. Selecting 메시지 produced SIGABRT in std::string(char const *) from onPageSelectedWithKey: (selection video, crash report). A long-press attempt reproduced the same abort at the selection boundary (video, crash report).
• After: iPhone 17 Pro simulator, iOS 26.5 (matching the reported major runtime). Unicode selection rendered the expected screen and repeated long-press gestures left the app alive with no new crash report (video, selected-state screenshot). Selection also passed on iOS 27.0 (screenshot).

XCUITest's synthesized hold selected the tab but did not fire the example's custom UILongPressGestureRecognizer; the same limitation occurred on the existing ASCII tab. The focused flow therefore verifies the original crash boundary by asserting that Unicode selection and long-press gestures keep the app alive and the correct tab visible. Full evidence notes and logs are in artifacts/issue-560.

Screenshots

Before interaction

After fix, selected on iOS 26.5