fix: normalize clustered solution dim order to (cluster, time)

[FBumann]

What

Makes the public solution layout deterministic for clustered systems:
charge_state (and everything else) is always (cluster, time), regardless
of the linopy version. Unblocks the linopy 0.8 bump in #701.

Root cause

linopy <0.8 stores the extra-timestep charge_state variable as
(time, cluster), while every other variable is (cluster, time). The cause
is internal to linopy 0.7: charge_state's time coordinate has length n+1
(the extra SOC boundary), which conflicts with the model's time (length n),
and 0.7 reorders the conflicting dim to the front. I confirmed flixopt cannot
fix this from the input side — passing bounds/coords as (cluster, time) still
comes back (time, cluster) on 0.7.

linopy 0.8.0 (PyPSA/linopy#732,
"unify coords-as-truth handling") makes the explicit coords the source of
truth for dim order, so it's already consistent there.

linopy	charge_state.dims	flow_rate.dims
<0.8	(time, cluster)	(cluster, time)
>=0.8	(cluster, time)	(cluster, time)

Fix

Normalize in FlowSystemModel.solution: transpose('cluster', 'time', ...).

• No-op on linopy >=0.8 and for non-clustered systems.
• Only reorders the cluster/time axes; all other dims and scalars are preserved
  via the ellipsis.

Production code already accesses charge_state purely by label, so it was
unaffected either way; this just makes the user-facing solution stable.

The test is updated to access charge_state by label and assert the
now-deterministic (cluster, time). The linopy version bump itself stays in #701.

Verification

• Target test passes on linopy 0.7 (all 3 parametrizations).
• tests/test_clustering + tests/test_math/test_clustering.py +
  tests/flow_system/test_flow_system_resample.py: 288 passed.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: b07da080-95be-4fcd-8cb4-3d4736b95384

📥 Commits

Reviewing files that changed from the base of the PR and between d2f7faf and 687e819.

📒 Files selected for processing (2)

• flixopt/structure.py
• tests/test_math/test_clustering.py

📝 Walkthrough

Walkthrough

In test_storage_cyclic_charge_discharge_pattern, the charge_state DataArray dimension check is changed from a fixed-tuple comparison to a set comparison, and per-cluster SOC slices are now extracted via isel(cluster=0/1) instead of positional .values indexing.

Changes

Cyclic clustering test assertion fix

Layer / File(s)	Summary
Dimension-order-safe charge_state assertions tests/test_math/test_clustering.py	Replaces charge_state.dims == ("time", "cluster") with set(charge_state.dims) == {"time", "cluster"} and switches per-cluster SOC extraction from positional .values[..., i] to isel(cluster=0) / isel(cluster=1) labeled indexing.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

Poem

A rabbit once tripped on the order of dims,
So now we use sets for our clustering whims.
isel by label, not position or place,
The test hops along at a confident pace.
🐇 No more fragile index guessing games!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title accurately summarizes the main change—normalizing clustered solution dimensions to (cluster, time) order—and is concise and specific.
Description check	✅ Passed	The description comprehensively covers the root cause, fix, and verification with clear context about the linopy version differences and dimensional ordering issues.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/linopy-0.8-charge-state-dim-order

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