docs(mvcc): proposal for concurrent writes via MVCC + BEGIN CONCURRENT

[joaoh82]

Summary

Adds docs/concurrent-writes-plan.md — a tentative Phase 10 design proposal that lifts SQLRite past SQLite's single-writer ceiling by adopting Turso's experimental MVCC approach: in-memory RowVersion chains, BEGIN CONCURRENT with snapshot isolation, and row-level write-write conflict detection at commit. Plan only — no code changes.

The doc covers:

• Why bother (Turso's reported up-to-4× write throughput uplift on disjoint-row workloads)
• What Turso does (MVCC index, read/write/commit protocols, WAL reuse, GC, current limitations)
• SQLRite's current concurrency model and the gap to bridge
• Proposed design — layered architecture + MvStore, RowVersion, ConcurrentTx data structures
• Phased plan — 9 sub-phases (10.1 multi-connection foundation → 10.9 docs sweep), each independently shippable
• SQL & API surface, file-format implications (WAL header v1 → v2; .sqlrite v5 unchanged), risks/non-goals, effort estimate, references

Tracking: SQLR-22 in Marvin.

Test plan

• Markdown renders cleanly on GitHub
• Plan to be reviewed before scheduling Phase 10
• No build / test impact (docs-only)

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