feat: modern event subsystem

[mdjastrzebski]

Summary

Merges the next/event-system feature branch (#1944, #1946) into main.

Adds an 'modern' event system for fireEvent and userEvent, opt-in with configure({ unstable_eventSystem: 'modern' }). It follows React Native's event dispatch: handlers receive a SyntheticEvent, and events bubble or go direct like on a device.

• Adds modern fireEvent, including fireEvent.press(), which calls the press callback of the element that becomes the touch responder (Pressable, Touchable*).
• Aligns modern userEvent.press() / longPress() with it, so both press the same element.
• 'legacy' stays the default and behaves as before. Its code moves to src/events/legacy/, with code used by both systems in src/events/shared/.
• Jest runs every test in both event systems (legacy and modern projects) against shared snapshots.

Test plan

• yarn validate (typecheck, tests in both Jest projects, lint, format check): 184 suites, 2278 tests, 518 snapshots passed.

[codecov]

Codecov Report

❌ Patch coverage is 99.87820% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.92%. Comparing base (0ecb9c9) to head (6b5a39a).

Files with missing lines	Patch %	Lines
src/events/modern/fire-event.ts	98.46%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1945      +/-   ##
==========================================
+ Coverage   98.56%   98.92%   +0.35%     
==========================================
  Files          87      105      +18     
  Lines        1745     2318     +573     
  Branches      478      610     +132     
==========================================
+ Hits         1720     2293     +573     
  Misses         24       24              
  Partials        1        1              

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