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feat(engine): Phase 11.3 MvStore skeleton + PRAGMA journal_mode (SQLR-22) (#124)

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

Third slice of Phase 11 (concurrent writes via MVCC + BEGIN
CONCURRENT). 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 rather than a lazy-init
dance.

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 instead of leaving a half-capped
  chain). `push_in_flight` for tests + 11.4's BEGIN CONCURRENT
  writes; in-flight versions invisible to other readers.
- `MvStoreError` enum: `NotCommitted`, `PreviousAlreadyCapped`,
  `PreviousCappedByInFlight`, `NonMonotonicBegin`.

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 val... (continued)

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