feat(engine): Phase 11.1 multi-connection foundation (SQLR-22)

[joaoh82]

Summary

First slice of Phase 11 — concurrent writes via MVCC + BEGIN CONCURRENT (SQLR-22). Refactors Connection from owning Database by value to holding Arc<Mutex<Database>>, so multiple Connection handles can address the same engine state from inside one process.

• New Connection::connect() mints a sibling handle that shares the backing Database. Connection: Send + Sync.
• Connection::database() / database_mut() (already #[doc(hidden)]) now return MutexGuard<'_, Database>. Internal call sites in src/ask/, the seven sqlrite-mcp tools that read the database, and the Python / Node / WASM SDK shims bind the guard to a local first.
• Six new tests in src/connection.rs: in-memory + file-backed sibling sharing, handle counting, 8-thread × 25-INSERT contention, per-handle prep cache, compile-time Send + Sync check.
• Docs: roadmap Phase 11 wrapper, design-decisions entry 12a, embedding.md "Sharing one database across threads" section, _index.md project-state line.

Scope honesty

Phase 11.1 is capability, not throughput. Every commit still serializes through the per-database Mutex and the existing pager flock. True multi-writer throughput on disjoint rows lands with BEGIN CONCURRENT in 11.4 — see the plan doc for the sequenced rest of Phase 11.

The plan doc internally numbers sub-phases as "Phase 10.x" — that's its working title from before Phase 10 (benchmarks) shipped. Roadmap entries say Phase 11.x; code comments and tests track the plan-doc numbering for continuity.

Test plan

• cargo build --workspace --exclude sqlrite-desktop --exclude sqlrite-python --exclude sqlrite-nodejs --exclude sqlrite-benchmarks --all-targets — clean
• cargo test --workspace --exclude sqlrite-desktop --exclude sqlrite-python --exclude sqlrite-nodejs --exclude sqlrite-benchmarks — 579/579 pass (32/32 connection tests, including 6 new)
• cargo clippy --workspace --exclude sqlrite-desktop --exclude sqlrite-python --exclude sqlrite-nodejs --exclude sqlrite-benchmarks --all-targets — no new warnings
• cargo fmt --all -- --check — clean
• cargo doc --workspace --exclude sqlrite-desktop --exclude sqlrite-python --exclude sqlrite-nodejs --exclude sqlrite-benchmarks --no-deps — no new warnings on changed files
• No file-format change; .sqlrite files written before this PR open identically

Followups (next sub-phases, separate PRs)

• 11.2 — MvccClock + ActiveTxRegistry; persist clock high-water in WAL header (v1 → v2)
• 11.3 — MvStore skeleton + PRAGMA journal_mode = mvcc snapshot-isolated reads
• 11.4 — BEGIN CONCURRENT writes + commit-time validation (the meat)
• 11.5 — Checkpoint integration + crash recovery
• 11.6 — GC (per-commit + background)
• 11.7 — Indexes under MVCC (deferred-by-design — separate slice)
• 11.8 — SDK + REPL propagation of Busy / BusySnapshot; new bench workload
• 11.9 — Docs sweep — promote plan to docs/concurrent-writes.md

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