oze: 未初期化 ScanRange union を修正 + -Werror=maybe-uninitialized を有効化 - #44

…alized

Phase 1 of the phased -Werror cleanup tracked in #43. Promotes one
warning class to error at a time and fixes its offenders in the same PR.

The fix
=======

cc/oze/include/transaction.hh:228 declared `uint64_t updated;` without
an initializer and then only `|=`'d into the upper or lower 32 bits in
two separate if/else branches. The other 32 bits stayed garbage, so the
value written back to ScanRange[index] via compare_exchange_strong was
not the intended `union(current [min,max], new [left,right])`.

Why this is an actual bug, not just a hygiene warning
-----------------------------------------------------

ScanRange is the fast-path bounding box read by the insert-validation
path at cc/oze/transaction.cc:851 — it splits the 64-bit atomic into
upper-32 = min, lower-32 = max and short-circuits the ScanHistory
linear walk if the inserted key falls outside [min, max]. Maintaining
that bounding box correctly requires it to grow monotonically:

  new_min = min(current_min, left)
  new_max = max(current_max, right)
  scan_range = (new_min << 32) | new_max

With `updated` uninitialized, the OR with garbage can:

  - shrink the apparent min and grow the apparent max, masking inserts
    that should have triggered the ScanHistory check (correctness:
    potential phantom anomaly), or
  - skew the range so the fast-path never short-circuits (perf only).

Either way, the behavior depends on stack contents, which is exactly
what -Wmaybe-uninitialized flagged. Initializing `updated = 0` lets the
two `|=` branches build the intended `(min << 32) | max` layout.

Paper cross-check
-----------------

The Oze PVLDB paper (Nemoto et al., PVLDB vol.18 p2321; extended
version at arxiv:2210.04179) describes phantom prevention via a per-
transaction scan history (txid + predicates) checked by inserters in
the validation phase. ScanRange is not in the paper — it is an
implementation-side fast-path filter in front of that scan history.
The fix preserves the paper's correctness contract (ScanHistory is
unchanged) while making the fast-path do what the surrounding code
clearly intends.

CMake side
==========

- `ccbench_add_protocol()` now adds `-Werror=maybe-uninitialized` to
  every protocol target. This is the first promotion in #43's phased
  rollout; the comment about deferring -Werror is replaced with one
  explaining the phased-promotion pattern.
- Verified: Debug+ASan and Release both build all 34 binaries clean.