feat: add multi RO transaction opening (ENG-2135)

[prestwich]

Summary

• Add begin_ro_sync_multi(n) and begin_ro_unsync_multi(n) to Environment for opening N read-only transactions guaranteed to share the same MVCC snapshot
• Add MdbxError::SnapshotDivergence for when the optimistic retry loop (16 retries) is exhausted under extreme write contention
• Enable safe parallel iteration over large tables without risking snapshot divergence between transactions

Implementation

Optimistic open-and-verify loop: opens N transactions, checks all txn_id() values match, drops and retries if a writer committed between opens. Generic helper begin_ro_multi deduplicates logic between sync/unsync variants.

Test plan

• Basic snapshot consistency (sync and unsync)
• Edge cases: n=0 returns empty vec, n=1 returns single tx
• Concurrent writer stress test: writer committing in a loop while reader opens multi-txn sets
• All existing tests pass
• Clippy clean (--all-features and --no-default-features)

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