Fix mfc wrapper script: add toolchain symlink and build directory

The wrapper script now:
1. Creates a toolchain symlink so mfc.sh can verify it's in MFC root folder
2. Creates a build directory with venv symlink for mfc.sh to write to
3. Prevents 'You must call this script from within MFC's root folder' error

This ensures the mfc command works properly when called from any location.

Author: sbryngelson

packaging/homebrew/mfc.rb

@@ -64,6 +64,7 @@ def install
     # Create smart wrapper script that:
     # 1. Works around read-only Cellar issue
     # 2. Activates venv automatically so cantera/dependencies are available
+    # 3. Sets up toolchain symlink so mfc.sh can find toolchain/util.sh
     (bin/"mfc").write <<~EOS
       #!/bin/bash
       set -e
@@ -80,6 +81,13 @@ def install
       cp "#{libexec}/mfc.sh" "$TMPDIR/"
       cd "$TMPDIR"
 
+      # Create toolchain symlink so mfc.sh can verify it's in MFC root folder
+      ln -s "#{prefix}/toolchain" toolchain
+
+      # Create build directory for mfc.sh to write to (with pre-existing venv symlink)
+      mkdir -p build
+      ln -s "#{venv}" build/venv
+
       # Run mfc.sh with all arguments
       exec ./mfc.sh "$@"
     EOS