Add doc comments to the four-bucket state model types

[martinjms]

Targets: epic/test-doc-pass
Depends on: #97
Epic: #95

[martinjms]

Opened PR #101 on branch feat/issue-98-add-doc-comments-to-the-four-bucket-stat (base: epic/test-doc-pass). Added doc comments to all public types in the replication_channel module — ChannelConfig, EntityStateDelta, EntityStateEntry, CloseReason, and IReplicationChannel — completing the four-bucket state model documentation pass. Tests pass. Format clean. Ready for review.