…#904)

Replace the (List<AbstractLoadedStage>, ExecutionResultBuilder) pair carried through the runner
with a single mutable aggregate (PipelineRun) that owns in-flight state and produces the
ExecuteResponseData.

- New `internal.core.pipeline.run`: **PipelineRun**, **StageRun**, **StageState** (polymorphic;
  Failed carries ErrorInfo).
- API change: `ExecutionPlanner.getNextExecution(List<AbstractLoadedStage>)` →
  `getNextExecution(PipelineRun)`. CommunityExecutionPlanner and CloudExecutionPlanner migrated.
- Intent-named mutators: `markStageStarted`, `markStageCompleted(result)`,
  `markStageFailed(exception)` — aggregate derives ErrorInfo from StageExecutionException.
- Removed **ExecutablePipeline** (trivial wrapper) and **ExecutionResultBuilder**;
  ExecutionPlan holds `List<ExecutableStage>` directly; `PipelineRun.toResponse()` builds the
  report.

No observable behavior change: runner still fail-fasts on StageExecutionException; response
shape and event firing points are identical.