Remove legacy semantics at linopy 1.0 (Stage 3)

[!NOTE]
The following was generated by AI.

Tracks the final Stage 3 of the v1 arithmetic-convention rollout (opt-in → default → 1.0): remove legacy semantics entirely, leaving only v1.

Prerequisite

• v1 is the default — #845. Legacy stays reachable via options["semantics"] = "legacy" from the default flip (a pre-1.0 minor) through to 1.0 — however many minors that spans; this issue removes it at 1.0. Removing a documented opt-out is a breaking change, so it rides the 1.0 major.

The strip (at 1.0)

A dependency-ordered, mostly-mechanical checklist already exists in
arithmetics-design/legacy-removal.md. grep "LEGACY: remove at 1.0" finds
every inline marker. In summary:

• Drop the semantics option, is_v1(), LinopySemanticsWarning, and every legacy warn/branch (config, semantics, expressions, variables, constraints).
• Collapse the *_v1 / *_legacy method pairs into single implementations.
• Remove the MultiIndex machinery kept alive only for legacy — the alignment.py level-projection subsystem, netcdf MI (de)serialize, assign_multiindex_safe, and the scattered isinstance(..., pd.MultiIndex) guards.
• Drop the legacy side of the test matrix (the pytest.mark.legacy cases and the dual-semantics parametrization).

Docs

• Re-frame arithmetics-design/convention.md / goals.md — drop the "v1"/legacy framing once there is only one convention.

Full checklist: arithmetics-design/legacy-removal.md.

Start with arithmetics-design/legacy-removal.md and run grep "LEGACY: remove at 1.0" to map the dependency-ordered work. Check the v1 default prerequisite in #845, then review the listed config, semantics, expressions, variables, constraints, alignment.py, netcdf MultiIndex handling, and tests. Done means legacy semantics and framing are removed and the remaining test matrix passes.