feat: Add fix(), unfix(), and fixed to Variable and Variables (#625)

* feat: Add fix(), unfix(), and fixed to Variable and Variables

Add methods to fix variables to values via equality constraints, with:
- Automatic rounding (0 decimals for int/binary, configurable for continuous)
- Clipping to variable bounds to prevent infeasibility
- Optional integrality relaxation (relax=True) for MILP dual extraction
- Relaxed state tracked in Model._relaxed_registry and restored by unfix()
- Cleanup on variable removal

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Add example to notebook covering fixing and dual extraction

* Add example to notebook covering fixing and dual extraction

* fix: Add type annotations to test_fix.py for mypy

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* docs: Add release note for fix/unfix/fixed feature

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* feat: Persist _relaxed_registry through netCDF IO

Serialize the relaxed registry as a JSON string in netCDF attrs so that
unfix() can restore integrality after a save/load roundtrip.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* refactor: Address PR review comments for fix/unfix feature

- Use as_dataarray() instead of DataArray() for value conversion
- Remove explanatory inline comments
- Move FIX_CONSTRAINT_PREFIX to constants.py
- Raise ValueError for out-of-bounds fix values instead of clipping
- Add overwrite parameter to fix() (default True)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* test: Parametrize fix tests across scalar and array data types

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* docs: Remove clipping from changelog for new feature

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: FBumann

doc/release_notes.rst

@@ -24,6 +24,7 @@ Upcoming Version
 * Improve handling of CPLEX solver quality attributes to ensure metrics such are extracted correctly when available.
 * Fix Xpress IIS label mapping for masked constraints and add a regression test for matching infeasible coordinates.
 * Enable quadratic problems with SCIP on windows.
+* Add ``fix()``, ``unfix()``, and ``fixed`` to ``Variable`` and ``Variables`` for fixing variables to values via equality constraints. Supports automatic rounding and optional integrality relaxation (``relax=True``) for MILP dual extraction.
 
 
 Version 0.6.5