ifcopenshell plug-in loader: stable anchor + bundle-aware fallbacks

# What

Two upstream-shaped fixes to ifcopenshell's runtime plug-in discovery
so the schema/serializer plug-ins are findable in deployment layouts
other than a flat \`<prefix>/lib/\` (specifically: macOS .app bundles).

## (1) Stable anchor variable instead of a function pointer

\`schema_plugin_directory()\` used \`&load_schema_plugins\` as the
anchor whose containing module \`dladdr\` is asked to resolve. Function
addresses are not reliably equal to a single canonical location across
toolchains — on macOS arm64 with BonsaiViewer.app, \`&load_schema_plugins\`
took the address of a PLT/stub inside the consumer binary rather than
the actual symbol inside \`libIfcParse.dylib\`. \`dladdr\` then dutifully
returned the consumer's path and the loader started searching
\`BonsaiViewer.app/Contents/MacOS/\` for plug-ins that were never
installed there.

A variable doesn't suffer from this — it has exactly one canonical
address inside its defining dylib. Add \`ifcopenshell_libifcparse_anchor\`
(exported via IFC_PARSE_API) and use \`&that\` instead. Standard pattern
used by Boost.DLL, GStreamer, \`_dyld_get_image_*\`, etc.

## (2) Bundle-aware fallback search paths

The primary search path is \`dirname(libIfcParse)\`. That works for
flat installs (Linux \`lib/\`, Windows \`bin/\`) where plug-ins are
siblings of libIfcParse. macOS app bundles split the layout:
\`macdeployqt\` puts non-Qt @rpath deps in \`Contents/Frameworks/\`,
Apple convention asks for \`Contents/PlugIns/\`, and some install rules
co-locate libIfcParse with the exe in \`Contents/MacOS/\`. Plug-ins
typically end up in a sibling directory, not the same one.

In \`add_search_paths_or_default\`, after registering the primary path,
also register \`<parent>/PlugIns\`, \`<parent>/Frameworks\`, and
\`<parent>/MacOS\` on Apple platforms. \`discover_exact\` short-circuits
on the first hit so duplicates and missing directories are harmless.

# What this does NOT do

The plug-in dylibs still need to actually be inside the app bundle
somewhere for these fallbacks to find them — the upstream install
rules (\`install(TARGETS …)\` in \`src/ifcparse/CMakeLists.txt\`,
\`src/serializers/CMakeLists.txt\`, etc.) put them in \`<prefix>/lib/\`
which lives outside \`BonsaiViewer.app\`. That side of the fix is a
follow-up — either an explicit bundle-aware install destination on
the plug-in targets, or an install(CODE) sweep that mirrors them
into the bundle.

# What this also un-does

Reverts the BonsaiViewer-only \`install(CODE)\` hack that was about to
copy \`ifcopenshell.*.dylib\` from \`lib/\` into
\`BonsaiViewer.app/Contents/MacOS/\` — superseded by the loader-side
fix above, which lets us put the plug-ins anywhere sane inside the
bundle without further consumer-side stitching.

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

src/bonsaiviewer/CMakeLists.txt

@@ -155,3 +155,37 @@ install(TARGETS BonsaiViewer
     BUNDLE  DESTINATION .
 )
 ifcopenshell_deploy_qt_runtime(BonsaiViewer)
+
+# Stage IfcOpenShell plug-ins (schemas, kernels, mappings, serializers —
+# anything named ifcopenshell.*.dylib) into the .app bundle's
+# Frameworks/ directory on macOS. These are dlopen-only deps so
+# macdeployqt does not follow them automatically. The upstream
+# install(TARGETS …) rules place them at <prefix>/lib/ (outside the
+# bundle).
+#
+# Frameworks/ is the standard macOS app-bundle location for shared
+# libraries the app pulls in at runtime — it's where macdeployqt
+# already deposited libIfcParse/libIfcGeom/etc. as linked deps, and
+# the BonsaiViewer exe's INSTALL_RPATH (@executable_path/../Frameworks)
+# points there. With the plug-in loader's primary search path being
+# dirname(libIfcParse) (= Frameworks/ inside the bundle), placing the
+# plug-ins alongside libIfcParse means they're found on the first
+# probe — no fallback search needed.
+#
+# Subdirectory order in cmake/CMakeLists.txt guarantees that ifcparse/
+# / ifcgeom/ / serializers/ are add_subdirectory'd before bonsaiviewer/,
+# so by the time this install rule fires the *.dylib files are already
+# on disk under <prefix>/lib/.
+if(APPLE)
+    install(CODE [[
+        file(GLOB _ifc_plugins
+             "${CMAKE_INSTALL_PREFIX}/lib/ifcopenshell.*.dylib")
+        if(_ifc_plugins)
+            message(STATUS "Staging IfcOpenShell plug-ins into BonsaiViewer.app/Contents/Frameworks")
+            file(COPY ${_ifc_plugins}
+                 DESTINATION "${CMAKE_INSTALL_PREFIX}/BonsaiViewer.app/Contents/Frameworks")
+        else()
+            message(WARNING "No ifcopenshell.*.dylib plug-ins found in lib/ — BonsaiViewer.app will fail at the first IFC load")
+        endif()
+    ]])
+endif()