Expose read-only update diagnostics

[HituziANDO]

Problem

Integrating applications cannot inspect when an update check was last attempted or succeeded, what version was received, or why the last check failed. Support teams therefore have little information for diagnosing update behavior.

Scope

• Expose a read-only diagnostics snapshot.
• Include last attempt, last success, last error category, current version, and last received store version.
• Keep state updates thread-safe and namespace-aware.
• Avoid storing response payloads or sensitive user data.
• Document persistence and reset behavior.

Dependencies

• Record daily version checks only after a successful lookup #14
• Extract UpdateCheckService and add a structured async API #23

Acceptance criteria

• Diagnostics accurately reflect success, failure, skip, and reset transitions.
• Reading diagnostics has no side effects.
• The snapshot is safe to read across concurrency domains.
• Applications can render or log diagnostics without parsing internal storage.

Tests

• Cover initial, attempted, successful, failed, skipped, namespaced, and reset states.

Relevant files

• Framework/Sources/SUK.swift
• Framework/Sources/SUKUserDefaults.swift