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feat(engine): Phase 11.9 WAL log-record durability + crash recovery (SQLR-22) (#130)

Pre-11.9, BEGIN CONCURRENT commits persisted *table state* through the
legacy save_database mirror, but MvStore's version index was reborn
empty on every reopen. That's correct for single-session workloads but
breaks down once cross-process MVCC matters — a second process could
hand out a begin_ts below an already-committed version's end and the
visibility rule would miscategorise one side.

11.9 closes that gap by adding a typed MVCC log-record frame on top of
the existing per-page WAL frames. The MVCC frame is appended before
the legacy save's commit-frame fsync, so a single fsync covers both:
a crash either keeps both writes or loses both — torn-write atomicity
for the whole transaction. On reopen, the WAL replay decodes every
MVCC frame and re-pushes the row versions into MvStore, then seeds
MvccClock past max(header.clock_high_water, max(commit_ts among
replayed batches)) so post-restart transactions can never regress.

Engine changes:
- src/mvcc/log.rs: MvccCommitBatch + MvccLogRecord types and codec
  ("MVCC0001" magic + commit_ts + record stream, fits one frame body).
- src/sql/pager/wal.rs: WAL_FORMAT_VERSION 2 → 3; MVCC_FRAME_MARKER =
  u32::MAX as the page-number discriminator; replay branches the
  frame stream into pending_mvcc that promotes onto recovered_mvcc on
  each commit barrier; Wal::append_mvcc_batch +
  Wal::recovered_mvcc_commits accessors.
- src/sql/pager/pager.rs: Pager proxies (append_mvcc_batch,
  recovered_mvcc_commits, clock_high_water, observe_clock_high_water).
- src/sql/pager/mod.rs: replay_mvcc_into_db drains recovered batches
  into Database::mv_store and seeds MvccClock at open time.
- src/connection.rs: commit_concurrent encodes the resolved write-set
  into an MvccCommitBatch, appends it pre-save_database, and bumps the
  in-memory WAL header's clock_high_water; six new tests cover
  round-trip persistence, multi-row batches, ROL... (continued)

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