feat: make output dimension names configurable via DimNames

[FBumann]

Closes #99.

Summary

The four structural output dims (cluster, timestep, period, segment) were hard-coded and reserved, forcing downstream wrappers (e.g. flixopt) to rename their own dims before/after every call — most painfully when a caller's slice dim is literally named period (multi-period optimization models).

This adds a frozen DimNames config, mirroring the existing ClusterConfig/ExtremeConfig/SegmentConfig style:

from tsam_xarray import aggregate, DimNames

aggregate(
    da,  # has a slice dim literally named "period"
    time_dim="time", cluster_dim="variable", n_clusters=8,
    dim_names=DimNames(period="original_period"),
)

• dim_names=None (default) reproduces today's names exactly.
• The resolved names are stored on ClusteringResult, so apply(), disaggregate(), and the JSON round-trip all reproduce them.
• Surfaced on AggregationResult via result.dim_names.

Changes

• _dim_names.py (new) — DimNames(cluster, timestep, period, segment); __post_init__ enforces uniqueness; as_tuple() helper.
• _core.py — aggregate(dim_names=None); threaded through _aggregate_single/_result_from_tsam and the _*_to_da builders. Replaced the blanket _RESERVED_DIMS set with _validate_dim_names, which validates the resolved output names against the actual input dims and raises on any collision.
• _clustering.py — ClusteringResult stores dim_names; all _build_* properties, apply/_apply_single, disaggregate/_disaggregate_single, and to_dict/from_dict use it. from_dict defaults missing keys to today's names for old blobs.
• _result.py — added result.dim_names; n_clusters/n_timesteps_per_period and the slice-view/disaggregate paths read the configured names.
• __init__.py — exports DimNames.
• docs/data-model.md — new "Custom output dimension names" section.
• test/test_aggregate.py — 10 new tests (period-collision resolved, apply/disaggregate/JSON round-trips, legacy-blob backcompat, uniqueness, segmentation).

Behavior-change note

segment is now part of the reserved/collision set. Previously _RESERVED_DIMS only held cluster/timestep/period, so an input dim named segment slipped through (a latent collision bug with segmented runs). Now a non-segmented run with an input dim literally named segment errors by default; pass DimNames(segment=...) to free it. Practically, segment as an output axis only labels ClusteringResult.segment_centers, which is None for all current tsam configs — so the reserve mainly closes the collision gap.

Verification

• pytest test/ — 537 passed, 4 skipped
• mypy src/ — clean
• ruff check / ruff format — clean

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Review details

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Configuration used: defaults

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Run ID: ea3eab29-360d-4073-b1b0-7274c2760fe6

📥 Commits

Reviewing files that changed from the base of the PR and between 5616d1c and 6514d66.

📒 Files selected for processing (7)

• docs/data-model.md
• src/tsam_xarray/__init__.py
• src/tsam_xarray/_clustering.py
• src/tsam_xarray/_core.py
• src/tsam_xarray/_dim_names.py
• src/tsam_xarray/_result.py
• test/test_aggregate.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/configurable-dim-names

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