read-only-viewer 0003: no lib catalog for viewers; 3D viewer from the session menu

- kicadShow3DViewer embind (pcbnew-only name, registered in the unguarded
  section so the merged kicad_editor image carries it): runOnCoroutine →
  ACTIONS::show3DViewer.
- Session menu: "3D viewer" row (pcbnew) — the only 3D entry once the wx
  chrome is hidden. Runs the deferred model prescan first.
- Read-only sessions: skip the boot-time enableRealtime scope-room socket
  (would 401 for non-members) and defer the board's 3D-model prescan until
  the viewer is opened (deferBoardModelPrescan / runDeferredModelPrescan).
- kicad submodule → read-only allowlist for the 3D viewer actions.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PXCntRWNn89M72EkrqMkvc

wasm/bindings/pcbnew_embind.cpp

@@ -1898,6 +1898,23 @@ void pcbCollabFitViewport( double aCx, double aCy, double aHalfW, double aHalfH
     presenceCore().fitViewport( aCx, aCy, aHalfW, aHalfH );
 }
 
+// JS → C++: open the board's 3D viewer (read-only-viewer / hide-UI sessions have
+// no wx View menu). Dispatched like a native edit — CallAfter + COROUTINE via
+// runOnCoroutine — so the tool runs on its own stack, where the 3D frame's
+// RunMainStack bounce (the 8/12 sleep-park fix) is valid; never from inside a
+// parked bridge crossing. Returns false when no board frame is up.
+bool pcbShow3DViewer()
+{
+    PCB_EDIT_FRAME* fr = pcbFrame();
+
+    if( !fr )
+        return false;
+
+    pcbjam_collab::runOnCoroutine(
+            fr, [fr]() { fr->GetToolManager()->RunAction( ACTIONS::show3DViewer ); } );
+    return true;
+}
+
 // JS pull of the current viewport transform (world↔screen mapping for the DOM layer):
 // `{cx,cy,scale,w,h}` — world center, pixels-per-IU scale, canvas size in px.
 std::string pcbCollabGetViewport()
@@ -2795,6 +2812,9 @@ EMSCRIPTEN_BINDINGS(pcbnew) {
     function("kicadLayersGetState", &pcbLayersGetState);
     function("kicadLayersSetVisible", &pcbLayersSetVisible);
     function("kicadLayersSetActive", &pcbLayersSetActive);
+    // Session-menu 3D entry (read-only-viewer 0003) — pcbnew-only name,
+    // merged-image safe (false when no board frame is up).
+    function("kicadShow3DViewer", &pcbShow3DViewer);
     // pcbnew-only test helper (no eeschema counterpart — name is not shared).
     function("kicadCollabTestItemBlob", &kicadCollabTestItemBlob);
     // pcbnew-only ysync-review repro hooks (names not shared with eeschema).