feat: add AddSeq for incremental sync

[requilence]

Summary

• Add per-space monotonically growing AddSeq counter to storage changes
• Enables incremental sync between space storage and CRDT storage: a consumer tracks its last processed seq and fetches only newer changes via GetAfterAddSeq, without rescanning the full tree
• Single atomic.Add(1) per AddAll/AddAllNoError call — all changes in one batch share the same seq
• Shared atomic.Uint64 counter across all tree storages in a space, set by spaceStorage
• spaceStorage.New initializes counter from max existing seq in DB on startup
• Gaps in seq are expected and safe (rolled-back transactions, validator noop counters) — only monotonic growth is guaranteed
• Seq is local to the peer, not synced across nodes

Changes

• StorageChange.AddSeq field, AddSeqKey constant, Storage.GetAfterAddSeq interface method
• storage.SetAddSeq / storageDeferredCreation.SetAddSeq wiring
• spaceStorage manages shared counter, injects into all tree storages
• objecttreevalidator uses a noop zero-counter for ephemeral validation storages

Test plan

• TestAddSeq_AddAll — batch seq assignment, increment across calls
• TestAddSeq_AddAllNoError — duplicate inserts tolerated, seq still increments
• TestAddSeq_NoopCounter — zero-valued counter works (validator path)
• TestAddSeq_GetAfterAddSeq — threshold queries return correct subsets
• TestAddSeq_GetAfterAddSeq_Empty — no false positives on empty results
• TestAddSeq_Parallel — 10 goroutines sharing one counter, no duplicate seqs
• TestAddSeq_DeferredCreation — seq propagated through lazy storage creation