`x - a <= 0` and `x <= a` build different constraints

[FBumann]

Issue Description

x - a <= 0 and x <= a are the same constraint, but linopy builds them differently in some cases. Subtraction aligns its operands by position when they share a same-size dimension — discarding coordinate labels — whereas the <= constraint operator aligns by label. The two forms disagree whenever the operands' labels are not already in the same order.

Reproducible Example

import pandas as pd
import linopy

m = linopy.Model()
x = m.add_variables(coords=[["A1", "A5", "A11", "A100"]], name="x")
coef = pd.Series([100, 1, 5, 11], index=["A100", "A1", "A5", "A11"])

print(x - coef <= 0)   # [A1]: x[A1] <= 100   <- positional, wrong
print(x <= coef)       # [A1]: x[A1] <= 1     <- by label, correct

coef["A1"] is 1, so the correct constraint is x[A1] <= 1. x <= coef produces that; x - coef <= 0 produces x[A1] <= 100 — it paired x[A1] with coef's first entry (A100), ignoring labels. Confirmed on linopy 0.7.0.

Expected Behavior

x - a <= 0 and x <= a are algebraically identical and must build the same constraint — either both align by label, or a label mismatch raises.

Context

Same positional / override-join behaviour as #586 and #670. #586's own snippet (add_constraints(x >= series)) takes the label-aligned path and no longer reproduces, but the bug is alive in arithmetic. The strict-alignment convention in #591 would fix it.