Constant SOURCE_OFFSET_WHOLE_RUN 

pub const SOURCE_OFFSET_WHOLE_RUN: i64 = i64::MAX; // 9_223_372_036_854_775_807i64

The chunk_seq sentinel meaning “the whole run at this offset is durably applied” — the common case (every pure-row run and every run-boundary commit). A genuine value < this is a mid-run checkpoint (offset, chunk_seq) left by the commit-at-DDL-boundary follower (RELAY-DDL-DESIGN.md §6.6): chunks 0..=chunk_seq of offset are applied, the tail is not. The resume/dedup compare is the keyset (offset, chunk_seq), with the incoming begin(R) treated as (R, WHOLE_RUN) — so a whole run sorts at/above any of its mid-run positions. i64::MAX is safe as a sentinel: chunk_seq is a 0-based within-run ordinal (one per spilled ≤CHUNK_ROWS chunk), so a real value reaching i64::MAX is physically impossible. Mirrors the relay fan-out’s ScanPos “past the end of this run’s chunks” sentinel (rindle-replicator).