build_osx: --shared + skip geometry-writer plug-ins (~3x bundle shrink)

Mirrors the Rocky workflow's two-part size reduction (27249770e
"Reduce Rocky package size") on macOS:

1. Pass `--shared` to nix/build-all.py. The default builds
   IfcOpenShell as static libs, which means every plug-in dylib
   (schemas × 8, kernels × 3, mappings × 8, writers × 8, document
   serializers × ~4, linework processing) statically embeds a full
   copy of libIfcParse + libIfcGeom. With --shared the plug-ins
   reference @rpath/libIfcParse.dylib + @rpath/libIfcGeom.dylib and
   the per-plug-in dylib drops from ~30-50 MB to a few MB each.
   Dominant size win.

2. Filter `ifcopenshell.geometry.writer.*.dylib` out of BonsaiViewer's
   plug-in staging step in src/bonsaiviewer/CMakeLists.txt. These are
   the per-schema OBJ / glTF / DAE / STP / IGS / SVG / TTL export
   converters — heavy because each one inlines the full schema, and
   BonsaiViewer is a viewer, never an exporter, so they're pure
   deadweight inside the bundle. Additive on top of --shared.

Bundle went 300 MB → expected ~100 MB, in line with Linux (~100 MB)
and Windows (~80 MB).

The IFCOPENSHELL_BUILD_PYTHON_WRAPPER=off gate is unchanged for now
— once we confirm BonsaiViewer.app size + functionality look sane,
we can ungate the Python wrapper and see if shared-builds-on-macOS
shake out its install issues too.

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

.github/workflows/build_osx.yml

@@ -88,10 +88,15 @@ jobs:
           # actually drops ifcopenshell.document.rdb.dylib into
           # site-packages/ifcopenshell/ (see the commit that added this
           # gate for the full history + the rocksdb/macdeployqt write-up).
+          # --shared mirrors build_rocky.yml after 27249770e: builds
+          # IfcOpenShell as shared libs so each plug-in dylib references
+          # libIfcParse / libIfcGeom via @rpath instead of statically
+          # embedding them — the dominant size win for BonsaiViewer.app
+          # (per-plugin libs go from ~30-50 MB to a few MB).
           CXXFLAGS="-O3" CFLAGS="-O3 ${DARWIN_C_SOURCE}" ADD_COMMIT_SHA=1 BUILD_CFG=Release \
             BUILD_BONSAIVIEWER=ON QT_DIR="${QT_DIR}" \
             IFCOS_BUILD_PYTHON_WRAPPER=off \
-            python3 ./nix/build-all.py -v --diskcleanup ${MAC_INTEL} \
+            python3 ./nix/build-all.py -v --diskcleanup --shared ${MAC_INTEL} \
             | tee build.log
 
       - name: Upload Build Logs