docs(mvcc): proposal for concurrent writes via MVCC + BEGIN CONCURRENT (#109)

Adds docs/concurrent-writes-plan.md — a Phase 10 design proposal modeled
on Turso's experimental MVCC, lifting SQLRite past SQLite's single-writer
ceiling. Tracked in Marvin (SQLR-22). No code changes; plan only.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: joaoh82