ifcviewer: section-plane cut tool on web — shared gizmo, true-face pick, drag/Del

Full section tool for the web viewport, with the gizmo + interaction shared with
desktop from one codebase.

- True-face surface pick. pickSurfaceAt had always ray-cast the instance AABB (to
  skip a depth readback), so cuts sat in front of the real surface. The pick
  fragment already computes the exact world_pos (it clips sections with it); now
  it OUTPUTS it to a 3rd pick MRT (RGBA32F) that every pick path renders, and
  pickSurfaceAt / pickSurfaceAtAsync read it back (decodeMappedPickPosition;
  ray-AABB kept only as a fallback). The web async pick chains id -> normal ->
  position spontaneous staging maps.
- Web tool: LMB drops a cut at the picked surface (LMB drag still orbits), K
  toggles, Shift+K clears; oriented to the real MRT surface normal. Exports + a
  Section / Clear cuts toolbar pair.
- Shared gizmo: lifted the section-gizmo renderer (SECTION_WGSL + thick-line AA +
  quad+arrow VBO + pack + screen-space hit-test) out of the Qt-coupled
  OverlayRenderer into a Qt-free SectionGizmoRenderer that ViewportCore::render
  draws for BOTH desktop and web (both already render via render()). One identical
  gizmo; OverlayRenderer's now-dead section code removed. Fixed 1 m size (matches
  the desktop constant).
- Interaction (shared): hitTestSectionGizmo (SectionGizmoRenderer::hitTest) +
  beginSectionDrag / updateSectionDrag / endSectionDrag live in ViewportCore.
  Drag a gizmo arrow to slide the plane along its normal; Del/Backspace removes
  the most recent cut. Desktop's ViewportWindow dropped its duplicate hit-test /
  drag math + state and delegates to the core; web wires the same calls.

Tests: sectionPlaneCount add/clear/cap (Catch2, 125); web smoke "click a surface
cuts geometry, clear restores" exercises the shared gizmo + 3-MRT pick (11/11).
Desktop object-pick / marquee unaffected; BonsaiViewer builds.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ifcviewer-web/CMakeLists.txt

@@ -104,7 +104,7 @@ target_link_options(IfcViewerWeb PRIVATE
     # EMSCRIPTEN_KEEPALIVE alone keeps the symbols in the binary but doesn't
     # add them to Module. ccall lets shell.html pass a JS string (the ?model
     # URL) to load_sidecar_from_url_c without manual heap marshalling.
-    "-sEXPORTED_FUNCTIONS=['_main','_raf_tick_c','_load_sidecar_from_source_c','_clear_scene_c','_ifcv_on_range_done','_ifcv_chunks_resident_c','_ifcv_chunks_total_c','_ifcv_model_count_c','_ifcv_model_resident_c','_ifcv_model_total_c','_ifcv_bytes_total_c','_ifcv_bytes_needed_c','_ifcv_bytes_loaded_c','_view_all_c','_frame_selection_c','_toggle_projection_c','_projection_is_ortho_c','_standard_view_c','_toggle_fly_c','_fly_is_active_c','_hide_selected_c','_isolate_selected_c','_show_all_c','_toggle_xray_c','_xray_is_active_c']"
+    "-sEXPORTED_FUNCTIONS=['_main','_raf_tick_c','_load_sidecar_from_source_c','_clear_scene_c','_ifcv_on_range_done','_ifcv_chunks_resident_c','_ifcv_chunks_total_c','_ifcv_model_count_c','_ifcv_model_resident_c','_ifcv_model_total_c','_ifcv_bytes_total_c','_ifcv_bytes_needed_c','_ifcv_bytes_loaded_c','_view_all_c','_frame_selection_c','_toggle_projection_c','_projection_is_ortho_c','_standard_view_c','_toggle_fly_c','_fly_is_active_c','_hide_selected_c','_isolate_selected_c','_show_all_c','_toggle_xray_c','_xray_is_active_c','_toggle_section_c','_clear_section_c','_section_is_active_c']"
     # ccall: shell.html passes the ?model URL string to load_sidecar_from_url_c.
     # HEAPU8: lets tooling/tests read the wasm heap size (e.g. to verify a large
     # sidecar streams by range instead of loading whole). Standard, zero-cost.