build_osx: build BonsaiViewer on macOS via build-all.py

Linux (build_rocky.yml) and Windows (win/build-all-win.py) already pass
BUILD_BONSAIVIEWER=ON when building from source; macOS was the odd one
out. Three small changes to bring it up to parity:

1. .github/workflows/build_osx.yml — \`brew install qt\` (Qt6 with Svg)
   in the Install Dependencies step, then set QT_DIR=\$(brew --prefix
   qt) and BUILD_BONSAIVIEWER=ON in the Run Build Script env.

2. nix/build-all.py — install_qt6() now honours a pre-set QT_DIR.
   Before this change get_qt6_aqt_config() raised on non-Linux,
   blocking bonsai builds on macOS/Windows from ever using a
   system-provided Qt6. We now validate that QT_DIR points at a real
   Qt6 install (probes lib/cmake/Qt6/Qt6Config.cmake) and skip the aqt
   download path if so. Linux flow is unchanged: when QT_DIR is unset
   the function falls through to the existing aqtinstall path.

build_osx.yml stays workflow_dispatch-only — slow run (~1h with
ccache, longer cold), so manual fire when wanted. Cherry-pick this
file + nix/build-all.py to v0.8.0 to make the workflow dispatchable
against the ifcviewer-wgpu branch before the squash lands.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.github/workflows/build_osx.yml

@@ -39,6 +39,10 @@ jobs:
           brew update
           # preinstalled: xz, cmake
           brew install git bison autoconf automake libffi findutils
+          # qt brings in Qt6 + Svg; nix/build-all.py honours pre-set
+          # QT_DIR so BonsaiViewer doesn't try to aqtinstall (which is
+          # Linux-only).
+          brew install qt
           echo "$(brew --prefix findutils)/libexec/gnubin" >> $GITHUB_PATH
           # Mac is using bison 2.5 by default, but we need 3.5+ for swig.
           echo "$(brew --prefix bison)/bin" >> $GITHUB_PATH
@@ -77,7 +81,9 @@ jobs:
             /usr/local/bin/brew install gettext openssl
           fi
           set -o pipefail
+          export QT_DIR="$(brew --prefix qt)"
           CXXFLAGS="-O3" CFLAGS="-O3 ${DARWIN_C_SOURCE}" ADD_COMMIT_SHA=1 BUILD_CFG=Release \
+            BUILD_BONSAIVIEWER=ON QT_DIR="${QT_DIR}" \
             python3 ./nix/build-all.py -v --diskcleanup ${MAC_INTEL} \
             | tee build.log