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feat(engine): Phase 11.6 MVCC garbage collection (SQLR-22) (#127)

Bounds in-memory MvStore growth. Without GC, every committed
version stays forever in the in-memory chain — a memory leak
that grows linearly with commits. Per-commit sweep + an
explicit Connection::vacuum_mvcc() for full-store drains
together cover the v0 model.

Promoted ahead of plan-doc 11.5 (checkpoint integration)
because durability already works through the legacy
save_database mirror (11.4). The unbounded-MvStore-growth gap
was the next concrete user-impact concern after 11.5 closed
the snapshot-read hole. Plan-doc 11.5 (checkpoint) → roadmap
11.7; plan-doc 11.6 (GC) → roadmap 11.6 (this slice).

New on MvStore:
- active_watermark() — returns min_active_begin_ts.unwrap_or
  (u64::MAX). Versions whose end is > watermark must be kept;
  ≤ watermark are reclaimable.
- gc_chain(row_id, watermark) — sweeps one row's chain,
  reclaims superseded versions, drops the row from the outer
  map if its chain becomes empty.
- gc_all(watermark) — full-store sweep. Snapshots row keys
  upfront so the outer map lock isn't held across per-chain
  locks.

Connection::commit_concurrent gains a per-commit GC sweep on
the write-set's chains. Crucially drops the TxHandle FIRST so
its begin_ts exits the active-tx registry — otherwise the
watermark would stay pinned to our own begin_ts and we'd
preserve every version we just wrote. With the handle dropped,
the watermark advances naturally and the sweep can reclaim
aggressively.

New Connection::vacuum_mvcc() public method runs a full-store
sweep at the current watermark. Returns the count reclaimed.
Safe to call regardless of journal_mode (no-op on Wal-mode
databases). The "vacuum the whole store" escape hatch for
memory-pressure workloads or tests that want a deterministic
baseline.

Tests (11 new):
- 7 in mvcc::store::tests:
  - active_watermark_is_max_with_no_readers
  - active_watermark_tracks_oldest_in_flight_tx
  - gc_chain_reclaims_versions_bel... (continued)

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Source Files on job 25652227157.1