feat: add kernel_reducers, framework engine_error, and Sidecar Clone

[OussamaSaoudi]

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• stack/c8 [Files changed]
  • stack/c1 [Files changed]
    • stack/c3 [Files changed]
      • stack/c5 [Files changed]
        • stack/c2 [Files changed]
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            • stack/c7 [Files changed]

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What changes are proposed in this pull request?

Adds the KernelReducer trait and the four canonical reducers (MetadataProtocol, CheckpointHint, SidecarCollector, and Handle) that drive log-replay aggregations. Each reducer is a small stateful visitor over a partition of input rows; KernelReducerToken plus the FinishedHandle/Extractor pair give the executor a typed channel for shipping finished reductions back to the SM driver.

Also lands:

• framework/engine_error.rs: the EngineError typed failure the engine surfaces to the SM (distinct from the kernel-to-caller DeltaError). Bundled here because handle.rs depends on it.
• actions/mod.rs + actions/visitors.rs: Sidecar: Clone, SidecarVisitor: Debug + Clone, and Protocol::try_new_from_data annotated
  #[internal_api]. Folded in because the kernel reducers depend on these (separating them out would be three trivial-LOC PRs).

Cross-sub-chain plans/mod.rs conflict expected when the basic-IR PR lands first: resolve by syncing this sub-chain after that lands. The state_machines module here declares only framework::engine_error; the remaining framework members (state_machine, coroutine, plan_context) land in follow-up PRs.

Depends on the errors PR for DeltaError (in chain ancestry).

How was this change tested?

[github-actions]

PR title does not match the required pattern. Please ensure you follow the conventional commits spec.

Your title should start with feat:, fix:, chore:, docs:, perf:, refactor:, test:, or ci:, and if it's a breaking change that should be suffixed with a ! (like feat!:), and then a 1-72 character brief description of your change.

Title: feat: add kernel_reducers, framework engine_error, and Sidecar Clone

[codecov]

Codecov Report

❌ Patch coverage is 83.51648% with 75 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.65%. Comparing base (721faee) to head (bddc0b4).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
kernel/src/plans/errors/mod.rs	59.37%	39 Missing ⚠️
...ernel/src/plans/kernel_reducers/checkpoint_hint.rs	82.81%	5 Missing and 6 partials ⚠️
kernel/src/plans/kernel_reducers/handle.rs	90.10%	8 Missing and 1 partial ⚠️
...nel/src/plans/kernel_reducers/metadata_protocol.rs	88.75%	4 Missing and 5 partials ⚠️
...nel/src/plans/kernel_reducers/sidecar_collector.rs	87.50%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2642      +/-   ##
==========================================
- Coverage   88.67%   88.65%   -0.02%     
==========================================
  Files         207      216       +9     
  Lines       65204    65906     +702     
  Branches    65204    65906     +702     
==========================================
+ Hits        57818    58428     +610     
- Misses       5173     5252      +79     
- Partials     2213     2226      +13     

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