feat: modern event subsystem

[mdjastrzebski]

Summary

Adds an experimental modern event system that follows React Native's own dispatch (capture and bubble phases, direct events). Turn it on with configure({ unstable_eventSystem: 'modern' }). The default stays 'legacy'.

• Moves the current implementation to src/events/legacy/ and adds the new one in src/events/modern/.
• fireEvent and userEvent choose the system on each call. fireEvent.press stays legacy for now.
• The whole test suite runs in both systems.

Test plan

yarn test:ci, yarn typecheck, yarn lint and yarn format:check pass in both systems. I haven't run it locally against older React Native versions.

[codecov]

Codecov Report

❌ Patch coverage is 99.55556% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.70%. Comparing base (5519b24) to head (ecccf77).

Files with missing lines	Patch %	Lines
src/events/modern/fire-event.ts	97.50%	1 Missing ⚠️
src/events/shared/update-native-state.ts	93.33%	1 Missing ⚠️

Additional details and impacted files

@@                  Coverage Diff                  @@
##           next/event-system    #1944      +/-   ##
=====================================================
+ Coverage              98.56%   98.70%   +0.13%     
=====================================================
  Files                     87       95       +8     
  Lines                   1744     2080     +336     
  Branches                 459      550      +91     
=====================================================
+ Hits                    1719     2053     +334     
- Misses                    24       26       +2     
  Partials                   1        1              

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