fix: add wgrib2 nix derivation and DataCase Req.Test shared mode

- nix/wgrib2.nix: build wgrib2 3.8.0 from source with NCEPLIBS-g2c 2.3.0,
  matching the Dockerfile.base build.
- flake.nix: wire wgrib2 derivation into dev shell so direnv loads it.
- DataCase: call Req.Test.set_req_test_from_context/1 in setup so
  async:false tests get shared Req.Test mode, making stubs visible
  to Tasks spawned by async_stream_nolink.

flake.nix

@@ -37,8 +37,8 @@
           system,
           ...
         }:
-        {
-          _module.args.pkgs = import nixpkgs {
+        let
+          pkgs' = import nixpkgs {
             inherit system;
             config = {
               allowUnfree = true;
@@ -57,9 +57,16 @@
             ];
           };
 
+          # wgrib2 is compiled from source — not in nixpkgs
+          wgrib2 = pkgs'.callPackage ./nix/wgrib2.nix { };
+        in
+        {
+          _module.args.pkgs = pkgs';
+
           # Development shell with PostgreSQL, Rust, Elixir LSP
-          devShells.default = pkgs.callPackage ./nix/shell.nix {
+          devShells.default = pkgs'.callPackage ./nix/shell.nix {
             pre-commit-hooks = inputs.pre-commit-hooks.lib.${system};
+            inherit wgrib2;
           };
 
           # Nix formatter for flake files

nix/wgrib2.nix

@@ -0,0 +1,90 @@
+{
+  stdenv,
+  lib,
+  cmake,
+  gfortran,
+  zlib,
+  libaec,
+  libpng,
+  openjpeg,
+  pkg-config,
+  fetchFromGitHub,
+  fetchurl,
+}:
+
+let
+  g2cVersion = "2.3.0";
+
+  g2c = stdenv.mkDerivation {
+    pname = "nceplibs-g2c";
+    version = g2cVersion;
+
+    src = fetchFromGitHub {
+      owner = "NOAA-EMC";
+      repo = "NCEPLIBS-g2c";
+      rev = "v${g2cVersion}";
+      hash = "sha256-9z0QipDqX5e/IgcK6y0q9ovja5jxwszLbWNUQQBIsss=";
+    };
+
+    nativeBuildInputs = [
+      cmake
+      pkg-config
+    ];
+    buildInputs = [
+      zlib
+      libaec
+      libpng
+      openjpeg
+    ];
+
+    cmakeFlags = [
+      "-DCMAKE_BUILD_TYPE=Release"
+      "-DUSE_PNG=ON"
+      "-DUSE_AEC=ON"
+      "-DUSE_OpenJPEG=ON"
+      "-DUSE_Jasper=OFF"
+    ];
+  };
+in
+stdenv.mkDerivation {
+  pname = "wgrib2";
+  version = "3.8.0";
+
+  src = fetchFromGitHub {
+    owner = "NOAA-EMC";
+    repo = "wgrib2";
+    rev = "v3.8.0";
+    hash = "sha256-cJR8CdkAWYVj54saYjigqncKCFXHpekWZ4yAMdYHYYg=";
+  };
+
+  nativeBuildInputs = [
+    cmake
+    gfortran
+  ];
+  buildInputs = [
+    zlib
+    libaec
+    libpng
+    openjpeg
+    g2c
+  ];
+
+  cmakeFlags = [
+    "-DCMAKE_BUILD_TYPE=Release"
+    "-DUSE_AEC=ON"
+    "-DUSE_G2CLIB_LOW=ON"
+    # Help CMake find the g2c library from our local build
+    "-DCMAKE_PREFIX_PATH=${g2c}"
+  ];
+
+  postInstall = ''
+    strip $out/bin/wgrib2
+  '';
+
+  meta = {
+    description = "Utility to read and write GRIB2 files";
+    homepage = "https://github.com/NOAA-EMC/wgrib2";
+    license = lib.licenses.gpl3Plus;
+    platforms = lib.platforms.linux;
+  };
+}

test/support/data_case.ex

@@ -32,6 +32,11 @@ defmodule Microwaveprop.DataCase do
     Microwaveprop.DataCase.setup_sandbox(tags)
     Microwaveprop.DataCase.reset_score_files()
     Microwaveprop.DataCase.stub_nexrad_default()
+
+    # sync tests (async: false) need shared Req.Test mode so stubs
+    # are visible to Tasks spawned by async_stream_nolink.
+    Req.Test.set_req_test_from_context(tags)
+
     :ok
   end