ifcviewer: first-person / fly view — share the desktop fly camera with web

DRY the fly camera into ViewportCore so desktop and web share identical math:
- flyMove(fwd,back,right,left,up,down,boost,dt): WASD in the view plane, QE
  along world +Z, forward = eye→target (no snap after orbiting), Shift = 5x,
  dt clamped to 0.1s. flyLook(dx,dy): turn in place (yaw/pitch, eye pinned,
  0.2 deg/px, pitch ±89.9). flyAdjustSpeed(notches): wheel scales speed x1.25.
  fly_move_speed_ + orbitEye now live in the core (orbitEye's duplicate removed
  from ViewportWindow).
- Desktop ViewportWindow: fpsIntegrate / mouse-look / wheel-speed call the core
  methods; behaviour unchanged, BonsaiViewer builds clean.
- Web main_web: Shift+F (or the Fly toolbar button) enters and pointer-locks the
  canvas; W/A/S/D/Q/E + Shift are held-tracked (keydown/keyup) and integrated
  each RAF frame with wall-clock dt; pointer-lock mouse deltas drive flyLook;
  wheel tunes speed. Exit on Esc OR a canvas click (matches desktop) — a
  pointerlockchange handler catches the browser eating the first Esc to release
  the lock (so a single Esc exits), guarded by fly_locked so a denied lock on
  entry doesn't insta-exit. Fly button reflects/ syncs active state.

Verified: web enter→WASD moves→click/Esc exits, 0 GPU errors; 113/113 desktop
core + 6/6 web smoke; desktop app 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']"
+    "-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']"
     # 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.