feat: consume upstream react-native-screens nested-scroll seam

[AmatoGiulio]

Goal

Consume the neutral Android nested-scroll delegate seam prototyped in AmatoGiulio/react-native-screens#1 without making react-native-screens depend on this package.

The goal is the generic transport path, not the Material3 reference UI.

Design

• keep the existing generic ReactNativeScreenNestedScrollBridge as the implementation;
• compile the react-native-screens adapter only when the installed screens project actually contains ScreenNestedScrollInterop.kt;
• keep the main Android source set completely independent of com.swmansion.rnscreens;
• leave react-native-screens 4.26.x behavior unchanged;
• install the optional adapter once from the standard RN package composition root;
• no per-frame reflection: reflection is used only once to discover the optional installer class;
• ship the optional source set in the npm tarball and assert that through check:package-surface;
• document the updated upstream design: current native-stack ScreensCoordinatorLayout + new Stack StackHeaderCoordinatorLayout, rather than patching Screen directly.

Upstream ownership

The screens prototype always runs react-native-screens' existing CoordinatorLayout/AppBar/BottomSheet behavior first. This package receives only the nested-scroll distance still available afterward. React Native remains the source touch/position/fling owner.

Validation

• react-native-screens fork lint JS: PASS
• react-native-screens fork typecheck: PASS
• react-native-screens fork Android lint / Spotless: PASS
• react-native-screens fork Android Fabric arm64-v8a build: PASS
• react-native-scroll-interop + forked react-native-screens Kotlin compile in a bare RN consumer: PASS
• external React Navigation native-stack runtime proof: PASS
  • no NativeScrollHost around the screen;
  • real ReactNestedScrollView bound through the screens seam;
  • Material3 reference consumer resolves as topBar=true and transaction becomes active=true;
  • touch scroll / reverse: PASS;
  • fling / reverse fling / non-touch momentum: PASS;
  • touch-to-momentum handoff reaches momentum-stop;
  • completed transaction conservation remains broken=0;
  • forward navigation binds the next screen source; Back restores the previous source and frontmost ownership;
  • no crash, jump or double consumption observed.

The debug ledger also reports some orphanPre entries where React Native emits a PRE callback without a corresponding POST callback. These are counted separately and are not broken completed frames; all completed frames in the runtime proof remained balanced (broken=0).

This PR remains draft and is not intended for merge or publish until an official react-native-screens release contains the seam. No version change and no npm publish.