Enhancement: Add AsyncSequence wrapper for fetchRecordChanges streaming

[leogdion]

Overview

Add an AsyncSequence-based API for streaming record changes, providing a more ergonomic alternative to manual pagination with fetchRecordChanges().

Current API

// Manual pagination
let result = try await service.fetchRecordChanges(syncToken: token)
if result.moreComing {
    let next = try await service.fetchRecordChanges(syncToken: result.syncToken)
}

// Or convenience method (loads all into memory)
let (records, token) = try await service.fetchAllRecordChanges()

Proposed API

// Stream records as they're fetched
for try await record in service.recordChangesSequence(syncToken: token) {
    process(record)
}

// Or with zone specification
for try await record in service.recordChangesSequence(
    zoneID: .init(zoneName: "Articles"),
    syncToken: token
) {
    process(record)
}

Benefits

• Memory efficient: Process records as they arrive without buffering
• Cancellable: Natural cancellation via AsyncSequence
• Ergonomic: Familiar Swift Concurrency patterns
• Backpressure: Consumer controls pacing

Implementation Notes

• Use AsyncThrowingStream internally
• Maintain sync token state between pages
• Handle moreComing flag automatically
• Support cancellation mid-stream
• Add tests for cancellation and error scenarios

Related

• Implemented in Fetching Database Changes (changes/database) #46 (fetchRecordChanges)
• Follow patterns from Swift Concurrency best practices