core/sqlite: speedup cache write using multi-vealue sqlite inserts

[karlicoss]

sqlite3.executemany() repeatedly steps and resets a single-row statement for every cached item.

Emit one multi-row INSERT per Cachew chunk instead, splitting at SQLITE_LIMIT_VARIABLE_NUMBER and caching SQL by batch size.

This preserves one row per item and atomic publication while reducing write overhead.

Benchmarks:

$ uv run --group testing --python /home/karlicos/.local/share/uv/python/cpython-3.14.4-linux-x86_64-gnu/bin/python3.14 -m pytest_benchmark compare .benchmarks/Linux-CPython-3.14-64bit/0008_sqlite3-backend.json .benchmarks/Linux-CPython-3.14-64bit/0009_sqlite-multivalue.json -k 'test_05_real_dump_e2e and sqlite' --between=mean,median --time-unit=ms --name=short

--------------------------------- benchmark 'datetimes-05_dump_e2e': 1 tests, 2 sources ----------------------------------
Name (time in ms)                                       0008 Mean  0009 Mean     ΔMean  0008 Median  0009 Median   ΔMedian
--------------------------------------------------------------------------------------------------------------------------
05_real_dump_e2e[cachew-real-sqlite-datetimes-100k]      269.2184   246.2861     -8.5%     266.2679     246.1437     -7.6%
--------------------------------------------------------------------------------------------------------------------------

---------------------------------- benchmark 'nested-dataclass-05_dump_e2e': 1 tests, 2 sources ---------------------------------
Name (time in ms)                                              0008 Mean  0009 Mean     ΔMean  0008 Median  0009 Median   ΔMedian
---------------------------------------------------------------------------------------------------------------------------------
05_real_dump_e2e[cachew-real-sqlite-nested-dataclass-100k]      183.3674   154.5616    -15.7%     183.3567     153.9058    -16.1%
---------------------------------------------------------------------------------------------------------------------------------

--------------------------------- benchmark 'primitive-int-05_dump_e2e': 1 tests, 2 sources ----------------------------------
Name (time in ms)                                           0008 Mean  0009 Mean     ΔMean  0008 Median  0009 Median   ΔMedian
------------------------------------------------------------------------------------------------------------------------------
05_real_dump_e2e[cachew-real-sqlite-primitive-int-100k]       71.8638    45.1905    -37.1%      72.0352      45.2104    -37.2%
------------------------------------------------------------------------------------------------------------------------------

--------------------------------- benchmark 'union-dataclass-05_dump_e2e': 1 tests, 2 sources ----------------------------------
Name (time in ms)                                             0008 Mean  0009 Mean     ΔMean  0008 Median  0009 Median   ΔMedian
--------------------------------------------------------------------------------------------------------------------------------
05_real_dump_e2e[cachew-real-sqlite-union-dataclass-100k]      166.0094   133.0951    -19.8%     165.6174     132.8846    -19.8%
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Legend:
  Cyan: reference source for comparison. Green: improvement, Red: regression.
  Δ: percentage change from reference source.