feat(engine): Phase 11.3 MvStore skeleton + PRAGMA journal_mode (SQLR-22)

[joaoh82]

Summary

Third slice of Phase 11 — concurrent writes via MVCC + BEGIN CONCURRENT (SQLR-22). Adds the in-memory version index (MvStore) and the per-database journal_mode toggle so 11.4 can wire the executor's read/write paths through a typed switch.

New sqlrite::mvcc::store

• RowID = (table, rowid), VersionPayload = Present(cols) | Tombstone, RowVersion { begin, end, payload }, RowVersionChain = Vec<RowVersion> behind Arc<RwLock<…>>.
• MvStore itself: Mutex<HashMap<RowID, Arc<RwLock<…>>>> plus a shared MvccClock and ActiveTxRegistry. The outer mutex guards the map's shape; the per-chain RwLock means readers walking different chains don't fight.
• MvStore::read(row, begin_ts) implements the textbook snapshot-isolation rule (begin <= T < end). Pure helper visible_at(version, begin_ts) exposed for tests + 11.6's GC.
• push_committed validates monotonic begin + caps the previous latest's end (validate-then-mutate; equal-begin retries surface as NonMonotonicBegin rather than leaving a half-capped chain). push_in_flight for tests + 11.4's BEGIN CONCURRENT writes; in-flight versions are invisible to other readers.
• MvStoreError enum covers NotCommitted, PreviousAlreadyCapped, PreviousCappedByInFlight, NonMonotonicBegin.

JournalMode + PRAGMA journal_mode

• New JournalMode { Wal, Mvcc } enum, default Wal. Database gains journal_mode, mvcc_clock: Arc<MvccClock>, mv_store: MvStore fields — all allocated on Database::new even in Wal mode so the toggle is cheap. set_journal_mode rejects Mvcc → Wal while the store carries committed versions (would silently strand them; v0 keeps this strict).
• PRAGMA journal_mode; (read) renders the current mode; PRAGMA journal_mode = wal | mvcc; (set) toggles. Case-insensitive on both pragma name and value; numeric values rejected; unknown modes return a typed error without disturbing the existing setting. Connection::journal_mode() is the public read accessor — every Connection::connect sibling sees the same value (per-database, not per-connection; see concurrent-writes-plan.md §8 for the open question).

Scope honesty

The executor doesn't consult MvStore yet. Routing reads through MvStore in this PR would surface as wrong rows because nothing publishes versions to it — the legacy commit path mutates Database.tables directly. 11.4 ships the read-path wiring AND the commit-path mirroring together because they're coupled. This PR delivers the data structure + toggle that 11.4 plugs into.

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 — 619/619 pass (24 new + 2 existing tests updated to use PRAGMA synchronous as the canary unknown-pragma name now that journal_mode is recognised)
• cargo clippy --workspace --exclude sqlrite-desktop --exclude sqlrite-python --exclude sqlrite-nodejs --exclude sqlrite-benchmarks --all-targets — no new warnings on changed files
• 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

New tests (24)

• mvcc::store::tests (12): visibility rule, cap-on-push, tombstone semantics, in-flight invisibility, monotonic-begin + in-flight rejection, tracked-rows + total-versions counts, Send + Sync compile check, concurrent-reads consistency, shared-clock pointer identity.
• mvcc::tests (3): JournalMode default, str round-trip, display.
• connection::tests (5): defaults_to_wal, set_to_mvcc_propagates_to_siblings, case-insensitive, rejects-unknown-value, rejects-numeric-value.

Followups

• 11.4 — BEGIN CONCURRENT writes, commit-time validation (write-write conflict → Busy / BusySnapshot), executor read path consults MvStore when journal_mode == Mvcc, legacy commit mirrors writes into MvStore.
• 11.5 — Checkpoint integration + crash recovery.
• 11.6 — Garbage collection.

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