issue: #51446
- message properties: add an explicit `Unreplicable` marker for concrete
WAL messages and expose builder opt-in through `WithUnreplicable`
- replication: skip unreplicable messages in the CDC sender and
secondary replicate interceptor before callback replay
- DDL producers: mark unsupported snapshot, manifest backfill, and
external collection refresh broadcasts as unreplicable
- streaming docs: document the message-level skip marker and current
unsupported DDL replication behavior
Signed-off-by: chyezh <chyezh@outlook.com>
