fix(prod-build): silence Nx.* undefined warnings in dev/test-only modules

Recalibrator and FrontalAnalysis use Nx for tensor math, but Nx is
declared `only: [:dev, :test]` in mix.exs because neither module has a
prod entry point — Recalibrator is invoked by hand from IEx and
FrontalAnalysis is research scaffolding. The prod compile would emit
"Nx.to_number/1 is undefined" warnings on every Nx call. Add a
`@compile {:no_warn_undefined, Nx}` directive to both modules so the
prod build is clean without dragging Nx/Axon/EXLA (~hundreds of MB) into
the runtime image.

lib/microwaveprop/propagation/recalibrator.ex

@@ -29,6 +29,13 @@ defmodule Microwaveprop.Propagation.Recalibrator do
 
   require Logger
 
+  # Nx is `only: [:dev, :test]` in mix.exs because the recalibrator is a
+  # research / weight-fitting tool that never runs in prod (no scheduled
+  # job, no cron, no LiveView entry point). Silence the prod-compile
+  # warnings about Nx.* being undefined — this module wouldn't be called
+  # without a developer explicitly invoking it.
+  @compile {:no_warn_undefined, Nx}
+
   @factor_keys ~w(humidity time_of_day td_depression refractivity sky season wind rain pwat pressure)a
 
   @doc """

lib/microwaveprop/weather/frontal_analysis.ex

@@ -21,6 +21,11 @@ defmodule Microwaveprop.Weather.FrontalAnalysis do
   All grid math uses Nx for vectorized 2D operations.
   """
 
+  # Nx is `only: [:dev, :test]` in mix.exs. This module is a research
+  # tool — no prod code path invokes it — so silence the prod-compile
+  # warnings about Nx.* being undefined.
+  @compile {:no_warn_undefined, Nx}
+
   @doc """
   Detect fronts from a 2D grid of surface temperature and pressure.