tombstone.zig ports upstream internal/tombstone: record tombstones (did/collection/rkey -> max superseding seq, from delete+update rows) and did tombstones (from sync rows and account rows decoding to active=false status=deleted), strict-inequality suppression of materialization rows only, marker rows never dropped, per-key max-seq merge, watermark eviction. keys cloned out of decoded block buffers. docs/upstream-compaction-spec.md condenses the upstream study: pass algorithm (chunked disk-window fold, issue #100 dual), watermark rules, rewrite crash safety (tmp+fsync+rename, block topology preserved), merge-tail vs steady, cache-invalidation design, port notes. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>