Add Event API

[Kyle-Ye]

Summary

Introduce the core Event type hierarchy:

• Event protocol (Identifiable, id/phase/timestamp requirements) + EventPhase enum (.began/.active/.ended/.failed).
• EventID struct — Hashable, CustomStringConvertible, Sendable wrapper around Int.
• SpatialEvent protocol (Event & LocationContaining).
• ScrollEvent protocol with delta / acceleratedDelta, plus ConcreteScrollEvent concrete struct.
• MouseEvent and TouchEvent concrete spatial events; MouseEvent.Button RawRepresentable struct (.primary/.secondary/.tertiary).
• Promote LocationContaining to public and add IdentifiableLocation<ID>.

Adds compatibility tests for EventID, MouseEvent, TouchEvent, ConcreteScrollEvent. The description tests are parametric with exact == assertions against the full NestedCustomStringConvertible output, covering every EventPhase case.

Test plan

• xcodebuild test -only-testing:OpenGesturesTests on iPhone 17 Pro / iOS 26.2 Simulator
• OPENGESTURES_COMPATIBILITY_TEST=0 xcodebuild test -only-testing:OpenGesturesCompatibilityTests on the same simulator
• OPENGESTURES_COMPATIBILITY_TEST=1 OPENGESTURES_USE_LOCAL_DEPS=1 xcodebuild test -only-testing:OpenGesturesCompatibilityTests on the same simulator (requires matching DarwinPrivateFrameworks branch)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.33333% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.19%. Comparing base (7422712) to head (6a01ad0).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
Sources/OpenGestures/Event/Event.swift	0.00%	2 Missing ⚠️
Sources/OpenGestures/Event/ScrollEvent.swift	77.77%	2 Missing ⚠️
Sources/OpenGestures/Util/LocationContaining.swift	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #13      +/-   ##
==========================================
+ Coverage   71.85%   74.19%   +2.33%     
==========================================
  Files          56       62       +6     
  Lines        1439     1519      +80     
==========================================
+ Hits         1034     1127      +93     
+ Misses        405      392      -13     

Flag	Coverage Δ
ios	70.19% <93.33%> (+2.99%)	⬆️
macos	74.19% <93.33%> (+2.33%)	⬆️

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