chore: checkpoint web Blender migration source, tools and docs

Accumulated uncommitted work: GHOST Web backend, WebGL/GPU web layer,
web workflow engine and protocol, tooling, docs and task records.
Large generated evidence, user-testing archives, build snapshots and
imported test assets remain untracked.

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

blender-5.2.0/intern/cycles/session/session.cpp

@@ -77,7 +77,9 @@ Session::Session(const SessionParams &params_, const SceneParams &scene_params)
   };
 
   /* Create session thread. */
+#ifndef __EMSCRIPTEN__
   session_thread_ = make_unique<thread>([this] { thread_run(); });
+#endif
 }
 
 Session::~Session()
@@ -93,8 +95,12 @@ Session::~Session()
   session_thread_cond_.notify_all();
 
   /* Destroy session thread. */
-  session_thread_->join();
+  if (session_thread_) session_thread_->join();
   session_thread_.reset();
+#ifdef __EMSCRIPTEN__
+  path_trace_->flush_display();
+  path_trace_->set_display_driver(nullptr);
+#endif
 
   /* Destroy path tracer, before the device. This is needed because destruction might need to
    * access device for device memory free.
@@ -113,6 +119,12 @@ Session::~Session()
 
 void Session::start()
 {
+#ifdef __EMSCRIPTEN__
+  if (!params.background) {
+    progress.set_error("Cycles interactive session requires Web thread scheduling; final CPU render is available");
+    return;
+  }
+#endif
   {
     /* Signal session thread to start rendering. */
     const thread_scoped_lock session_thread_lock(session_thread_mutex_);
@@ -123,6 +135,16 @@ void Session::start()
     session_thread_state_ = SESSION_THREAD_RENDER;
   }
 
+#ifdef __EMSCRIPTEN__
+  try {
+    thread_render();
+  }
+  catch (...) {
+    session_thread_state_ = SESSION_THREAD_WAIT;
+    throw;
+  }
+  session_thread_state_ = SESSION_THREAD_WAIT;
+#endif
   session_thread_cond_.notify_all();
 }