refactor(int32): per-model label dtype instead of sticky process-global

[FBumann]

This implements int dtype control per Model instead of through a global config option (global kept).
This resolves all potential issues with the global for little complexity.
I think this is the right approach.

Note

The following content was generated by AI.

Stacked on #822. Keeps its auto-widening behavior but moves the label dtype from the sticky process-global option onto the Model itself.

What changed

• The label dtype becomes a Model() argument: Model(label_dtype=np.int64), default np.int32, exposed as Model.label_dtype. The linopy.options["label_dtype"] setting is removed entirely — embedders forward constructor kwargs instead (PyPSA: n.optimize(model_kwargs={"label_dtype": np.int64})), so config.py returns to its pre-perf: default integer arrays to int32 for ~25% memory reduction #566 state.
• A model widens only itself to int64 when its labels would exceed the int32 maximum; the UserWarning recommends passing label_dtype=np.int64 upfront.
• All cast sites read the dtype from their object's model reference; common.save_join takes it as an explicit fill_dtype parameter instead of reading the global.
• read_netcdf restores the widened dtype per model (from the counters, as before). Pickle round-trips preserve it automatically since it is an instance attribute.
• OptionSettings.widen_label_dtype() and its sticky-reset() semantics are removed, along with the test-suite fixture that guarded against global-state leakage.

Why per-model

#822 chose the process-global because "the four float→int cast sites … have no reliable model reference (expressions can be detached)". That premise doesn't hold in the current codebase: BaseExpression.__init__ raises unless model is a Model instance, and every label_dtype cast site is a method on an object with a validated model reference (BaseExpression, Variable, Variables, Constraints, CSRConstraint). The only model-less site, save_join, is called with integer_dtype=True from exactly three container properties that all have self.model in scope.

With a reliable model reference, a per-model monotonic widen gives the same no-narrowing guarantee at the same zero hot-path cost (an attribute read), and removes the global design's edge cases:

• No revert footgun. Under the global design, following the documented revert (options["label_dtype"] = np.int32) while a widened model is alive lets the cast sites silently narrow >2^31 labels — int64 → int32 .astype() wraps to negative values without any error. Per-model, there is nothing to revert.
• Widening travels with the model. A widened model shipped to another process via pickle/cloudpickle (dask, multiprocessing) keeps its dtype; the global flag does not cross process boundaries.
• No cross-model contamination. perf(int32): auto-widen labels to int64 on overflow instead of raising #822 documents that one huge model permanently costs every later small model in the process its int32 memory/speed win. Per-model, small models are unaffected.
• One knob in one place: the constructor argument. Nothing needs test isolation.

Cast-site → model-reference verification

Site	Enclosing scope	Model access
expressions.py vars cast in init	BaseExpression.__init__	model param, enforced isinstance(model, Model) (check moved above the cast)
expressions.py sanitize	BaseExpression	self.model
expressions.py simplify (2×)	LinearExpression	self.model
variables.py ffill/bfill/sanitize	Variable	self.model (init enforces isinstance(model, Model))
variables.py flat remap	Variables	self.model
constraints.py _to_dataset (2×)	CSRConstraint	self._model
constraints.py flat remap	Constraints	self.model
common.py save_join	free function	dtype passed by callers (Variables.labels, Constraints.labels, Constraints.vars)

Tests

test/test_dtypes.py: the widen tests assert other models stay int32; new test_label_dtype_init_arg (+ invalid-dtype rejection), test_widen_applies_to_expressions, test_auto_widen_survives_pickle; the netcdf test asserts the loaded model is widened. The restore_label_dtype fixture and the option tests are gone. Full suite passes.

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