[codex] fix persisted change log semantics

[pdurlej]

Summary

• return persisted change log rows as LoggedChange with the stored semantic ChangeType() instead of always returning UnknownChange
• preserve /items outer-array server indexes on flattened Item values
• use preserved server indexes when processing and logging changes, so multi-entity server items do not drift in change_log.server_index
• add regression tests for logged change types and multi-entity server-index handling

Context

Sync() returns typed changes from fresh processing, but the persisted query APIs (ChangesSince, ChangesSinceIndex, ChangesForEntity) previously routed through scanChangeLog, which always returned UnknownChange. This made the stored change APIs much less useful for clients like OpenClaw.

This patch intentionally avoids pretending to reconstruct full concrete structs like TaskTitleChanged{OldTitle: ...} from existing rows, because the current schema stores only metadata plus the raw item payload. LoggedChange preserves the real stored change type and metadata without inventing fields that were not persisted.

The patch also fixes a related indexing issue: History.Items flattens each server item map into multiple Item values, but processItems used the flattened slice offset as the server index. Multiple entities from one server item can now share the same persisted server index.

Validation

• go test ./...