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/ifcparse/schema.cpp

@@ -205,8 +205,20 @@ ifcopenshell::plugin::metadata ifcopenshell::schema_plugin_metadata(const std::s
 	return metadata;
 }
 
+// Stable anchor symbol inside libIfcParse for plugin::module_directory()'s
+// dladdr/GetModuleHandleEx lookup. A variable (vs. a function pointer) has
+// exactly one canonical address inside its defining module — function
+// pointers can resolve to a PLT/stub copy in the consumer binary on some
+// toolchains (observed on macOS arm64: `&load_schema_plugins` resolved
+// inside BonsaiViewer.exe instead of libIfcParse.dylib, so dladdr returned
+// the exe's directory and the plugin search path ended up at
+// BonsaiViewer.app/Contents/MacOS/ instead of wherever libIfcParse — and
+// therefore the schema plug-ins — actually lived).
+extern "C" IFC_PARSE_API char ifcopenshell_libifcparse_anchor;
+IFC_PARSE_API char ifcopenshell_libifcparse_anchor = 0;
+
 std::filesystem::path ifcopenshell::schema_plugin_directory() {
-	return plugin::module_directory(reinterpret_cast<const void*>(&ifcopenshell::load_schema_plugins));
+	return plugin::module_directory(&ifcopenshell_libifcparse_anchor);
 }
 
 void ifcopenshell::load_schema_plugins(schema_registry& registry) {