feat: serialize HKWorkoutEvent.metadata in workout queries

[sh940701]

Summary

This PR adds metadata serialization to WorkoutEvent, making per-event metadata from HKWorkoutEvent available to JS consumers.

Closes #332

Changes

packages/react-native-healthkit/src/types/Workouts.ts

Added optional metadata field to WorkoutEvent interface using AnyMap from react-native-nitro-modules, consistent with how BaseObject.metadata is typed:

export interface WorkoutEvent {
  readonly type: WorkoutEventType
  readonly startDate: Date
  readonly endDate: Date
  readonly metadata?: AnyMap  // new
}

packages/react-native-healthkit/ios/WorkoutProxy.swift

Pass event.metadata through the existing serializeMetadata() helper in the events computed property:

return WorkoutEvent(
  type: type,
  startDate: event.dateInterval.start,
  endDate: event.dateInterval.end,
  metadata: serializeMetadata(event.metadata)  // new
)

Motivation

Apple's HKWorkoutEvent has a metadata: [String: Any]? property. For swimming workouts, Apple documents that HKMetadataKeySwimmingStrokeStyle should be "set on workout lap events". Without event metadata, consumers cannot access per-lap stroke style through the documented path.

Backward compatibility

• metadata is optional (AnyMap?) — existing consumers are unaffected
• Reuses the existing serializeMetadata() helper (already used for workout metadata, sample metadata, and route metadata)
• serializeMetadata() handles nil input gracefully (returns empty map)

Note

I haven't run bun codegen locally since Nitrogen may need to regenerate types for the new metadata field on WorkoutEvent. Happy to update if the generated output needs adjustment — just let me know.

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commit: c99a57f

[sh940701]

Closing to resubmit with a clean commit history — the branch had intermediate commits that shouldn't be in the final PR.