Operators align coordinates inconsistently (`*` by label, `+` `-` `/` by position)

[FBumann]

Issue Description

linopy's binary operators disagree on how to align coordinates when two operands share a same-size dimension:

Operator	Alignment
*	by label
+, -, /	by position — coordinate labels discarded (override join)
<=, >=, == (constraint creation)	by label

Because some operators align by label and others by position, algebraically identical expressions produce different results.

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"])
# coef["A1"] == 1  (coef's first entry is A100 == 100)

(x + coef).const.sel(dim_0="A1")     # 100   positional — should be 1
(x - coef).const.sel(dim_0="A1")     # -100  positional — should be -1
(x * coef).coeffs.sel(dim_0="A1")    # 1     by label   — correct
(x / coef).coeffs.sel(dim_0="A1")    # 0.01  positional — should be 1

Broken equivalences

Mixing a positional operator with a by-label one makes identical forms disagree:

• x - a <= 0 ≠ x <= a — - positional, <= by label (x - a <= 0 and x <= a build different constraints #707).
• x / a ≠ x * (1 / a) — / positional, * by label: x / coef pairs x[A1] with 1/100; x * (1/coef) pairs it with 1/1.

Expected Behavior

Every operator should align coordinates the same way — by label — and a label mismatch should raise rather than silently pair by position.

Context

Confirmed on linopy 0.7.0. Related: #586, #670, #707. The strict-alignment convention in #591 makes all operators align by label uniformly.