[kans]

Stacked on #1206. Third pebble.Engine split: the 15 methods from merge_surface.go, merge_accessor.go, rollback_expansion.go, expand_write_path_stats.go move to Expansion, same shape as Digests and SyncRuns. Bodies unchanged beyond e. → x.e..

Two things differ from the plan's table. synthLayer and the four write counters stay on Engine: grants.go writes them, so they are shared with the record path, not owned here; ExpandWritePathStats reads them through x.e. And Expansion carries the raw Pebble surface the compactor uses (NewIter, Get, IngestSSTs, NewFoldBatch, DropKeyRange, ReplaceRangeWithSSTs, Metrics, EstimateDiskUsage, LogCompactionMetrics, UnsafeForTesting, PebbleEngine) because those lived in merge_surface.go / merge_accessor.go. If that name reads wrong for the compactor's use, it is a rename, not a re-split.

Callers go through Expansion(): pebble_store.go (RollbackExpansion), adapter_reader.go (NewIter), and pkg/synccompactor/pebble/{compactor,kway,merge,overlay}.go. No forwarders, no new pebbleStore methods, export status unchanged.

Checks: go build ./..., go vet ./..., golangci-lint clean; go test ./pkg/dotc1z/... -count=1 ok including TestWriteMuHolders; focused -run 'Expan|Merge|Rollback|Synth|Layer' across pkg/sync and pkg/synccompactor ok.