Flink 1.18: dual-line CI, upstream suite and release validation

Current behavior

CI and the upstream suite already run both Flink lines. What remains is the release half: the
release workflow publishes one line. The Flink 1.18 benchmark is already recorded in #198.

Shipped

• The CI Java matrix runs the full suite on both lines, and uploads per-line portable SQL audit
  outcomes.
• All nine tagged connector and format module suites run on both lines.
• Lake module CI runs Paimon Parquet and ORC on both lines; Delta stays on 2.2.
• Image integration builds the optimized payload and runs the Session-cluster smoke test on both
  lines, including bin/check-artifacts.sh --flink-line.
• The upstream suite matrix runs runtime, formats, parquet, orc, kafka, paimon and
  state on both lines, with delta excluded on 1.18 and a separate Delta 3.3 / Flink 1.18 host
  audit job. Each run records result totals and separates native execution, expected fallback and
  cases with no execution contract.
• The loader is tested on both lines with real planner installation.

The optimized release+mimalloc Flink 1.18 Nexmark comparison shipped in #198 and is linked from
docs/benchmarks.md: recorded method and results.
It keeps the unchanged rowwise generator, blackhole sink and both transposes. All ten measured
queries are slower than stock (0.28–0.88x); the existing Kafka headline table is unchanged.

Remaining

• Publish and validate both artifact sets from the release workflow: separate Maven staging,
  line-qualified archives and image tags, per-line source and Javadoc artifacts, and
  bin/check-artifacts.sh against each line's artifact set. The workflow currently performs one
  deployment and assembles unsuffixed archives.
• Update docs/releasing.md with the completed automated dual-line publication process. The
  experimental build/package commands and benchmark methodology are already documented.

Publication of the 1.18 line stays blocked on #188 regardless of this issue; passing the CI and
upstream matrix does not establish real-cluster cross-line upgrade support.

Done when

The release workflow publishes and validates both lines' artifact sets. The required release-build
1.18 benchmark and its methodology are already recorded.

Split from #182.

Start with the release workflow and compare its current single deployment and unsuffixed archive assembly with the dual-line requirements in the issue. Trace bin/check-artifacts.sh for validation of each artifact set, then update docs/releasing.md with the completed process. Done means both lines publish separate Maven, archive, image, source, and Javadoc artifacts and each set is validated.