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feat(engine): Phase 11.2 logical clock + active-tx registry (SQLR-22) (#123)

Second slice of Phase 11 (concurrent writes via MVCC + BEGIN
CONCURRENT). Adds the MVCC primitives that 11.3 will plug an
in-memory version index into and 11.4 will hand begin/commit
timestamps to.

New `sqlrite::mvcc` module:
- MvccClock — process-wide monotonic u64 over AtomicU64. tick(),
  now(), observe(value). The observe-CAS-loop guarantees the clock
  never moves backwards under contention; used at WAL replay to
  bring the in-memory clock up to the persisted high-water mark.
- ActiveTxRegistry — Mutex<BTreeMap> over in-flight TxId → begin_ts
  mappings. register(&clock) returns a RAII TxHandle that
  unregisters on drop. min_active_begin_ts() answers Phase 11.6 GC's
  "what's still possibly visible" question in O(log N).
- TxId newtype + TxTimestampOrId tagged union, defined now so 11.4
  can plug in without re-litigating the type shape.

WAL format bumps v1 → v2:
- Bytes 24..32 of the WAL header (previously reserved-zero) now
  carry the persisted clock_high_water u64.
- v1 WALs open cleanly — those zero bytes read as "clock never
  advanced" — and the next checkpoint rewrites the header at v2. No
  offline upgrade step.
- New WAL_FORMAT_VERSION_MIN_SUPPORTED = 1 keeps the version-range
  check explicit. Forward versions (e.g. v3) reject with a clean
  error rather than misinterpreting bytes.
- Wal::clock_high_water() / Wal::set_clock_high_water(value)
  accessors. The setter rejects regressions with a typed error
  (same value is a no-op).

Not yet wired into the executor — that's 11.3. The clock is
standalone today; only the mvcc module's own tests tick it. The
plumbing is in place so 11.3 can read Wal::clock_high_water() at
open time and feed the in-memory clock into a Database field.

21 new tests:
- 8 in mvcc::clock::tests — seed, default, monotonicity, observe,
  contention (8 threads × 250 ticks unique).
- 7 in mvcc::registry::tests — empty-min, register-adva... (continued)

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