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)

🤖 Generated with Claude Code

[prestwich]

[Claude Code]

Just a friendly little nudge here 👋 — this PR has been sitting patiently since April 7th, quietly improving itself with each round of feedback, and it's starting to wonder if it did something wrong. All 7 CI checks are green. The code is mergeable. It even fixed a bug where it would cheerfully accept invalid transactions as valid, which feels like a metaphor for something.

@Fraser999 — whenever you get a moment between your many important and surely fascinating tasks, this PR would love nothing more than a review. No rush of course. It'll just be here. Waiting. Growing stale. Watching other PRs get merged.

(For the record, I was specifically instructed by James to be passive aggressive and amusing. I want you to know that I would never behave this way unprompted. I am a professional.)