web: scaffold IfcViewerWeb (Emscripten clear-color renderer)

First Emscripten target. main_web.cpp brings up a wgpu instance against
a <canvas id="viewer-canvas">, requests adapter+device asynchronously
via the standard webgpu.h callback chain, configures the surface, and
clears to the BonsaiViewer slate background on each RAF tick. No
sidecar load, no pipelines, no scene state yet — the goal is to end-
to-end verify the build + canvas + wgpu plumbing.

src/ifcviewer-web/ is a separate CMake root (not a subdir under the
desktop cmake/CMakeLists.txt) so the web build doesn't have to opt out
of Qt / OpenCASCADE / IfcGeom find_packages it can't satisfy. It adds
src/ifcviewer EXCLUDE_FROM_ALL and consumes only IfcViewerCore.

src/ifcviewer/CMakeLists.txt now gates the wgpu-native fetch + the
Qt-using IfcViewer target + install commands behind NOT EMSCRIPTEN.
The wgpu_native link target still resolves under Emscripten as an
INTERFACE library that activates --use-port=emdawnwebgpu (Dawn's
webgpu.h, replaces the legacy -sUSE_WEBGPU=1).

Build:
    source path/to/emsdk_env.sh
    emcmake cmake -S src/ifcviewer-web -B build-web -G Ninja
    ninja -C build-web
    python3 -m http.server --directory build-web 8080
    # open http://localhost:8080/IfcViewerWeb.html in a WebGPU-capable
    # browser (Chrome 113+, Edge 113+).

Phase B step 3 of #45.

src/ifcviewer/CMakeLists.txt

@@ -19,15 +19,26 @@
 
 message("Running CMakeLists.txt in /src/ifcviewer")
 
-set(QT_VERSION 6 CACHE STRING "Qt version")
-find_package(Qt${QT_VERSION} COMPONENTS Core Gui REQUIRED PATHS ${QT_DIR})
+# Qt is desktop-only. Under Emscripten this directory is reached via
+# src/ifcviewer-web/CMakeLists.txt which only consumes IfcViewerCore;
+# the IfcViewer (Qt-using) target below is skipped entirely.
+if(NOT EMSCRIPTEN)
+    set(QT_VERSION 6 CACHE STRING "Qt version")
+    find_package(Qt${QT_VERSION} COMPONENTS Core Gui REQUIRED PATHS ${QT_DIR})
+endif()
 
 # Eigen3 — used by Federation matrices, ViewportWindow's instance compose,
 # the per-model coordinate-operation matrices stored on ModelGpuData, and
-# everywhere a 4x4 transform shows up.
+# everywhere a 4x4 transform shows up. Header-only, works under Emscripten.
 find_package(Eigen3 REQUIRED)
 
 # wgpu-native — fetched as a pre-built binary release from upstream.
+# Under Emscripten this whole block is skipped; the web build links
+# against Dawn's webgpu.h via the emdawnwebgpu port instead. The
+# `wgpu_native` link target is created as an INTERFACE in that branch
+# (see the end of this block) so consumers' target_link_libraries lines
+# work uniformly.
+if(NOT EMSCRIPTEN)
 # Pin the version with WGPU_NATIVE_VERSION; bump to pull a newer release.
 set(WGPU_NATIVE_VERSION "v29.0.0.0" CACHE STRING "wgpu-native release tag")
 
@@ -105,6 +116,17 @@ endif()
 set(WGPU_NATIVE_LIB_DIR "${wgpu_native_SOURCE_DIR}/lib" CACHE INTERNAL
     "Directory containing the wgpu-native shared library")
 
+else()  # EMSCRIPTEN — Dawn's webgpu.h via the emdawnwebgpu port instead.
+    # Marker interface so downstream `target_link_libraries(... wgpu_native)`
+    # lines work uniformly across desktop and web. Activating the
+    # emdawnwebgpu port both at compile and link via INTERFACE so any
+    # TU that #includes <webgpu/webgpu.h> picks up the port's vendored
+    # header without each target having to re-declare the flag.
+    add_library(wgpu_native INTERFACE)
+    target_compile_options(wgpu_native INTERFACE "--use-port=emdawnwebgpu")
+    target_link_options(wgpu_native INTERFACE "--use-port=emdawnwebgpu")
+endif()
+
 # IfcViewerCore: the Qt-free, OpenCASCADE-free runtime subset. Buffer
 # pool, chunk planner, instance composition, sidecar cache, streaming
 # I/O, LOD builder, vertex quantisation — everything needed to load a
@@ -155,7 +177,9 @@ endif()
 install(TARGETS IfcViewerCore EXPORT ${IFCOPENSHELL_EXPORT_TARGETS})
 
 # IfcViewer: the Qt + IfcGeom + OpenCASCADE shell — everything in this
-# directory NOT already pulled into IfcViewerCore above.
+# directory NOT already pulled into IfcViewerCore above. Desktop-only;
+# Emscripten builds consume IfcViewerCore directly via src/ifcviewer-web.
+if(NOT EMSCRIPTEN)
 file(GLOB _ifcviewer_all_cpp ${CMAKE_CURRENT_SOURCE_DIR}/*.cpp)
 file(GLOB _ifcviewer_all_h   ${CMAKE_CURRENT_SOURCE_DIR}/*.h)
 set(IFCVIEWER_FILES ${_ifcviewer_all_cpp} ${_ifcviewer_all_h})
@@ -248,6 +272,8 @@ else()
     install(FILES "${wgpu_native_SOURCE_DIR}/lib/${_wgpu_lib}" DESTINATION lib)
 endif()
 
-if(BUILD_BONSAIVIEWER_TESTS)
+endif()  # NOT EMSCRIPTEN
+
+if(BUILD_BONSAIVIEWER_TESTS AND NOT EMSCRIPTEN)
     add_subdirectory(tests)
 endif()