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/source/blender/blenloader/intern/writefile.cc

@@ -342,8 +342,10 @@ bool ZstdWriteWrap::open(const char *filepath)
   }
 
   /* Leave one thread open for the main writing logic, unless we only have one HW thread. */
+#if !defined(__EMSCRIPTEN__) || defined(__EMSCRIPTEN_PTHREADS__)
   int num_threads = max_ii(1, BLI_system_thread_count() - 1);
   BLI_threadpool_init(&threadpool, ZstdWriteBlockTask::write_task, num_threads);
+#endif
   BLI_mutex_init(&mutex);
   BLI_condition_init(&condition);
 
@@ -393,7 +395,9 @@ void ZstdWriteWrap::write_seekable_frames()
 
 bool ZstdWriteWrap::close()
 {
+#if !defined(__EMSCRIPTEN__) || defined(__EMSCRIPTEN_PTHREADS__)
   BLI_threadpool_end(&threadpool);
+#endif
   tasks.free_no_destruct();
 
   BLI_mutex_end(&mutex);
@@ -418,6 +422,11 @@ bool ZstdWriteWrap::write(const void *buf, const size_t buf_len)
   task->frame_number = num_frames++;
   task->ww = this;
 
+#if defined(__EMSCRIPTEN__) && !defined(__EMSCRIPTEN_PTHREADS__)
+  write_task(task);
+  MEM_delete(task);
+  return !write_error;
+#endif
   BLI_mutex_lock(&mutex);
   BLI_addtail(&tasks, task);
 
@@ -1302,7 +1311,7 @@ static void write_id_placeholder(WriteData *wd, ID *id)
 }
 
 /** Keep it last of `write_*_data` functions. */
-static void write_libraries(WriteData *wd, Main *bmain)
+static void write_libraries(WriteData *wd, Main *bmain, const bool materialize_linked_ids)
 {
   const bool is_undo = wd->use_memfile;
 
@@ -1327,7 +1336,13 @@ static void write_libraries(WriteData *wd, Main *bmain)
 
     /* Gather IDs that are somehow directly referenced by data in the current blend file. */
     Vector<ID *> ids_used_from_library;
-    if (is_undo) {
+    if (materialize_linked_ids) {
+      /* Read-only SceneIR projection of the already loaded Main. Include indirect data too;
+       * Object.data, materials and nested collections must resolve to complete native IDs.
+       * This stream is never used as a saved blend or an undo checkpoint. */
+      ids_used_from_library = ids;
+    }
+    else if (is_undo) {
       /* Always write placeholders for all linked IDs in undo case. This allows to properly remove
        * linked data that should not exist on undo/redo. See also #read_undo_move_libmain_data,
        * #read_libblock_undo_restore_linked and #read_undo_libraries_cleanup_unused_ids. */
@@ -1417,7 +1432,7 @@ static void write_libraries(WriteData *wd, Main *bmain)
     /* Write placeholders for linked data-blocks that are used, and real IDs for the packed linked
      * ones. */
     for (ID *id : ids_used_from_library) {
-      if (ID_IS_PACKED(id)) {
+      if (materialize_linked_ids || ID_IS_PACKED(id)) {
         write_id(wd, id);
       }
       else {
@@ -1448,7 +1463,11 @@ extern "C" char build_hash[];
  * - for forward compatibility, `curscreen` has to be saved
  * - for undo-file, `curscene` needs to be saved.
  */
-static void write_global(WriteData *wd, const int fileflags, Main *mainvar)
+static void write_global(WriteData *wd,
+                         const int fileflags,
+                         Main *mainvar,
+                         Scene *current_scene,
+                         ViewLayer *current_view_layer)
 {
   const bool is_undo = wd->use_memfile;
   FileGlobal fg;
@@ -1464,6 +1483,10 @@ static void write_global(WriteData *wd, const int fileflags, Main *mainvar)
   fg._pad1 = nullptr;
 
   current_screen_compat(mainvar, is_undo, &screen, &scene, &view_layer);
+  if (!scene && current_scene) {
+    scene = current_scene;
+    view_layer = current_view_layer ? current_view_layer : BKE_view_layer_default_view(scene);
+  }
 
   /* XXX: still remap `G`. */
   fg.curscreen = screen;
@@ -1564,6 +1587,10 @@ BLO_Write_IDBuffer::BLO_Write_IDBuffer(ID &id, const bool is_undo, const bool is
   }
   temp_id->us = 0;
   temp_id->icon_id = 0;
+#ifdef WITH_WEB
+  // Dependency evaluation and undo-dirty tags are runtime state in ordinary Web snapshots.
+  if (!is_undo) { temp_id->recalc = 0; temp_id->recalc_after_undo_push = 0; }
+#endif
   temp_id->runtime = nullptr;
   /* Those listbase data change every time we add/remove an ID, and also often when
    * renaming one (due to re-sorting). This avoids generating a lot of false 'is changed'
@@ -1735,7 +1762,10 @@ static bool write_file_handle(Main *mainvar,
                               const int write_flags,
                               const bool use_userdef,
                               const BlendThumbnail *thumb,
-                              std::ostream *debug_dst)
+                              std::ostream *debug_dst,
+                              const bool materialize_linked_ids = false,
+                              Scene *current_scene = nullptr,
+                              ViewLayer *current_view_layer = nullptr)
 {
   WriteData *wd;
 
@@ -1758,7 +1788,7 @@ static bool write_file_handle(Main *mainvar,
   write_blend_file_header(wd);
   write_renderinfo(wd, mainvar);
   write_thumb(wd, thumb);
-  write_global(wd, write_flags, mainvar);
+  write_global(wd, write_flags, mainvar, current_scene, current_view_layer);
 
   /* The window-manager and screen often change,
    * avoid thumbnail detecting changes because of this. */
@@ -1784,7 +1814,7 @@ static bool write_file_handle(Main *mainvar,
   }
 
   /* Write libraries about libraries and linked data-blocks. */
-  write_libraries(wd, mainvar);
+  write_libraries(wd, mainvar, materialize_linked_ids);
 
   /* So changes above don't cause a 'DNA1' to be detected as changed on undo. */
   mywrite_flush(wd);
@@ -2124,14 +2154,36 @@ bool BLO_write_file_mem(Main *mainvar, MemFile *compare, MemFile *current, const
 bool BLO_write_file_to_callback(Main *mainvar,
                                 const int write_flags,
                                 BLO_WriteFileCallback callback,
-                                void *user_data)
+                                void *user_data,
+                                const bool materialize_linked_ids_for_snapshot,
+                                Scene *current_scene,
+                                ViewLayer *current_view_layer)
 {
   if (callback == nullptr) {
     return false;
   }
+  /* Reject foreign or retired pointers before emitting any bytes. */
+  if (current_scene) {
+    bool scene_found = false;
+    for (Scene &scene : mainvar->scenes) {
+      if (&scene == current_scene) scene_found = true;
+    }
+    if (!scene_found) return false;
+    if (current_view_layer) {
+      bool layer_found = false;
+      for (ViewLayer &view_layer : current_scene->view_layers) {
+        if (&view_layer == current_view_layer) layer_found = true;
+      }
+      if (!layer_found) return false;
+    }
+  }
+  else if (current_view_layer) {
+    return false;
+  }
   CallbackWriteWrap callback_wrap(callback, user_data);
   const bool err = write_file_handle(
-      mainvar, &callback_wrap, nullptr, nullptr, write_flags, false, nullptr, nullptr);
+      mainvar, &callback_wrap, nullptr, nullptr, write_flags, false, nullptr, nullptr,
+      materialize_linked_ids_for_snapshot, current_scene, current_view_layer);
   return (err == 0);
 }
 
@@ -2288,6 +2340,8 @@ void BLO_write_generated_pointer_tag(BlendWriter *writer, const void *data)
     return;
   }
   if (!BLO_write_is_undo(writer)) {
+    /* Generated storage can reuse an earlier ID's temporary allocation. */
+    writer->wd->stable_address_ids.pointer_map.remove(data);
     return;
   }
   const uint64_t address_id = get_address_id_for_undo(*writer->wd);