Upstream: adopt v1 arithmetic convention (PyPSA/linopy#591)

[FBumann]

Why

Without a predictable arithmetic contract from linopy, linopy_yaml cannot guarantee consistent behavior. A YAML expression like sum(p * cost, over=generator) compiles differently depending on operand order, coordinate alignment, and NaN handling:

• Positional alignment ignores labels (Constraints do not match RHS based on coordinates PyPSA/linopy#586, #550, #257)
• Subset constants break associativity
• User NaN silently swallowed with inconsistent fill values
• Variable vs LinearExpression paths give different results (Inconsistent multiplication behavior: Variable vs LinearExpression with subset coordinates PyPSA/linopy#569, #570)

We park stability on the upstream v1 convention refactor rather than working around these in our parser/builder.

Upstream tracking

• Milestone: Predictabe Arithmetics, Broadcasting and Joining
• Umbrella issue/PR: refac: Harmonize linopy operations and introduce a new predictable and strict convention PyPSA/linopy#591 — introduces linopy.options["arithmetic_convention"] = "v1" with strict coordinate matching, explicit NaN handling, and formal algebraic law tests.

Action for linopy_yaml once v1 is released

1. Opt into v1 globally: linopy.options["arithmetic_convention"] = "v1" in from_yaml() (or at module import).
2. Un-xfail the dispatch tests and verify they pass.
3. Document in SPEC.md that linopy_yaml requires the v1 convention.
4. Drop any workarounds that exist to paper over legacy-convention quirks.

Sub-issues that directly affect linopy_yaml today

• Constraints do not match RHS based on coordinates PyPSA/linopy#586 — Constraints do not match RHS based on coordinates (likely root of the current dispatch xfail)
• Inconsistent multiplication behavior: Variable vs LinearExpression with subset coordinates PyPSA/linopy#569 / #570 — Inconsistent multiplication with subset coordinates (relevant to p * cost patterns)
• Inconsistent coordinate override in arithmetic expressions PyPSA/linopy#257 — Inconsistent coordinate override in arithmetic expressions (relevant to sum(...) helpers)
• Inconsistent creation of dimensions in add_variables PyPSA/linopy#450 — Inconsistent creation of dimensions in add_variables

Close when

linopy ships the v1 convention as a released version, linopy_yaml has migrated, and the dispatch xfail markers are removed.