[FLINK-39781][table] Add LateralSnapshotJoinOperator with two-phase LOAD/JOIN execution

[fhueske]

What is the purpose of the change

Stateful keyed two-input operator backing LATERAL SNAPSHOT joins.

• Two phases (LOAD/JOIN) gated by an operator UnionListState; mixed-phase rescale collapses to LOAD. Flip triggers: build-side WM reaches loadCompletedTime, or a processing-time idle-timeout fallback.
• Probe records are buffered in LOAD and joined on flip. Build-side changes are buffered per-key and applied lazily in event-time order on the next per-key access once the build-side WM advanced, preserving atomic -U/+U visibility.
• Watermarks: build-side absorbed; probe-side held back in LOAD, forwarded in JOIN. NULL equi-keys filtered via JoinConditionWithNullFilters.
• State TTL via keyed processing-time timers with a post-flip grace window; rearms are amortized so effective time-to-eviction is in [1.0×, 1.5×] stateTtlMs.

Brief change log

• add LateralSnapshotJoinOperator according to FLIP-579

Verifying this change

• LateralSnapshotJoinOperatorTest

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): yes
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? no (operator is not accessible yet)
• If yes, how is the feature documented? n/a

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Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Co-authored-by: Claude Code 2.1.148 (Opus 4.7)

[fhueske]

⚠️ PR is not ready for review yet.

[flinkbot]

CI report:

• f189418 Azure: FAILURE

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