New _ReducedFlowSystemBuilder class (lines 82-381): (#593)

_ReducedFlowSystemBuilder
  ├── __init__(fs, aggregation_results, timesteps_per_cluster, dt, dim_names)
  │   └── Pre-computes: n_clusters, is_segmented, cluster_coords, time_coords, base_coords
  ├── _expand_and_combine()          # Shared pattern: expand_dims → combine_slices
  ├── build_cluster_weights()        # Extract weights from results
  ├── build_typical_periods()        # Extract and reshape cluster representatives
  ├── build_segment_durations()      # Extract segment durations (if segmented)
  ├── build_metrics()                # Extract RMSE/MAE metrics
  ├── build_reduced_dataset()        # Assemble variables into Dataset
  └── build()                        # Main entry: orchestrates all builders → FlowSystem

  Call sites updated:
  - cluster() → _ReducedFlowSystemBuilder(fs, ...).build(ds)
  - apply_clustering() → _ReducedFlowSystemBuilder(fs, ...).build(ds)

  Deleted from TransformAccessor (~360 lines):
  - _accuracy_to_dataframe() (moved to module-level)
  - _build_cluster_weight_da()
  - _build_typical_das()
  - _build_segment_durations_da()
  - _build_clustering_metrics()
  - _build_reduced_flow_system()
  - _build_reduced_dataset()

  Benefits:
  - Coordinates computed once in __init__, not passed to every method
  - Common _expand_and_combine() pattern extracted (was repeated 4x)
  - Clear single entry point: builder.build(ds)
  - Each build method independently testable
  - TransformAccessor significantly simplified

Author: FBumann