Add Singular integration attribute

[anglinb]

Summary

• Adds singularDeviceId to IntegrationAttribute
• Covers serialization in attribution tests
• Adds changelog entry

Verification

• git diff --check

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[greptile-apps]

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[pullfrog]

✅ No new issues found.

Reviewed changes — adds a singularDeviceId (Singular SDID) attribute to the public IntegrationAttribute API.

• Add singularDeviceId enum case — appended to IntegrationAttribute in IntegrationAttribute.swift, with the matching CustomStringConvertible arm returning "singularDeviceId". The case is added at the end of the Int-backed enum, so existing raw values stay stable and no wire format breaks.
• Test coverage — AttributionTests.swift adds .singularDeviceId to the serialization fixture and asserts storedProps["singularDeviceId"].
• Changelog — Enhancements entry noting the new attribute.

The CustomStringConvertible switch in IntegrationAttribute.swift is the only exhaustive switch over the enum, and it's updated. Attributes are serialized to the backend by their string description, so there is no raw-value decode or persisted-ordinal path the appended case could break. Clean, minimal, and correctly tested.

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