Epic: FieldValue TIMESTAMP correctness (write tagging + complex-type validation)

[leogdion]

Parent tracking issue for making CloudKit type/value handling correct on both the write and read paths.

Children

• FieldValueRequest can't tag scalar TIMESTAMP — Date/Time writes fail with BAD_REQUEST #375 — FieldValueRequest can't tag scalar TIMESTAMP → Date/Time writes fail with BAD_REQUEST (bug, priority: high — do first)
• Extend type/value contradiction validation to complex declared types (REFERENCE/ASSET/LOCATION/LIST) #376 — Extend type/value contradiction validation to complex declared types (REFERENCE/ASSET/LOCATION/LIST)

#376 builds on the strict-validation foundation from #375.