Pin NumPy to <2.0 for ArviZ 0.18 compatibility

ArviZ 0.18 requires numpy<2.0 but conda-forge's recent NumPy 2.0
migration caused CI failures. This pins NumPy to 1.x to restore
compatibility with the current ArviZ constraint (<=0.18).

Fixes daily CI failures that started on Nov 7, 2025.

Author: claude

CondaPkg.toml

@@ -3,3 +3,4 @@ pandas = ""
 matplotlib = ""
 xarray = ""
 arviz = ">=0.15.0,<=0.18"
+numpy = ">=1.23,<2.0"