docs: explain iOS 26 keyboard avoidance

[thiagobrez]

Summary

• document why iPhone Liquid Glass tab bars overlay full-bleed scene content on iOS 26+
• show the supported useBottomTabBarHeight pattern for bottom-fixed KeyboardAvoidingView content while preserving a full-screen background
• add a runnable example and durable before/after validation evidence
• leave Android unchanged because its separate IME behavior is already addressed by fix(android): keep BottomNavigationView pinned under the keyboard in edge-to-edge #549

The initial global scene-resizing approach was rejected during adversarial review: it would break iPad/sidebar/tvOS placement, full-bleed and translucent backgrounds, minimized bars, bottom accessories, and existing consumer offsets. The final change is intentionally consumer-scoped documentation and an example.

Validation

• yarn lint (passes with 3 existing warnings)
• yarn typecheck
• yarn build
• yarn build:ios
• standalone Xcode simulator build with Xcode 27 beta 4
• yarn build:android
• iPhone 17 Pro on iOS 26.5: input moves from the covered y=803.67 baseline to y=720.67–770, above the tab bar at y=791; semantic tap and keyboard-open text entry pass
• iPhone 17 Pro on iOS 27.0: same at-rest geometry and semantic tap pass
• Pixel 10 Pro Android baseline recorded to confirm the separate fix(android): keep BottomNavigationView pinned under the keyboard in edge-to-edge #549 scope

The repository-wide yarn test command has a pre-existing package with no tests, and the focused Jest fallback hits the existing React Native/Babel setup parse failure. This change adds no runtime library logic requiring a new unit seam.

Closes #505