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>

tools/web/build-webengine-alembic.py

@@ -0,0 +1,59 @@
+#!/usr/bin/env python3
+"""Build Blender's pinned Alembic for the existing Web CacheFile reader."""
+
+import hashlib
+from pathlib import Path
+import re
+import subprocess
+import tarfile
+import urllib.request
+
+
+ROOT = Path(__file__).resolve().parents[2]
+BASE = ROOT / "build_web_alembic"
+OCIO = ROOT / "build_web_ocio/install"
+
+
+def main():
+    versions = (ROOT / "blender-5.2.0/build_files/build_environment/cmake/versions.cmake").read_text()
+    version = re.search(r"set\(ALEMBIC_VERSION ([^)]+)\)", versions)[1]
+    expected = re.search(r"set\(ALEMBIC_HASH ([^)]+)\)", versions)[1]
+    algorithm = re.search(r"set\(ALEMBIC_HASH_TYPE ([^)]+)\)", versions)[1].lower()
+    if not (OCIO / "lib/cmake/Imath/ImathConfig.cmake").is_file():
+        raise SystemExit("Reuse/build the Web OCIO Imath installation first")
+    BASE.mkdir(exist_ok=True)
+    source = BASE / ("alembic-" + version)
+    if not (source / "CMakeLists.txt").is_file():
+        archive = BASE / ("alembic-" + version + ".tar.gz")
+        if not archive.is_file():
+            with urllib.request.urlopen(
+                "https://codeload.github.com/alembic/alembic/tar.gz/" + version,
+                timeout=60,
+            ) as response:
+                data = response.read()
+            if hashlib.new(algorithm, data).hexdigest() != expected:
+                raise SystemExit("Alembic download differs from Blender's pinned source hash")
+            archive.write_bytes(data)
+        if hashlib.new(algorithm, archive.read_bytes()).hexdigest() != expected:
+            raise SystemExit("Alembic archive differs from Blender's pinned source hash")
+        with tarfile.open(archive) as package:
+            package.extractall(BASE, filter="data")
+    toolchain = ROOT / ".emsdk/upstream/emscripten/cmake/Modules/Platform/Emscripten.cmake"
+    subprocess.run([
+        "cmake", "-S", str(source), "-B", str(BASE / "build"), "-G", "Ninja",
+        "-DCMAKE_TOOLCHAIN_FILE=" + str(toolchain),
+        "-DCMAKE_BUILD_TYPE=Release", "-DCMAKE_INSTALL_LIBDIR=lib",
+        "-DCMAKE_INSTALL_PREFIX=" + str(BASE / "install"),
+        "-DCMAKE_PREFIX_PATH=" + str(OCIO), "-DCMAKE_FIND_ROOT_PATH=" + str(OCIO),
+        "-DImath_DIR=" + str(OCIO / "lib/cmake/Imath"),
+        "-DCMAKE_CXX_FLAGS=-fexceptions", "-DALEMBIC_SHARED_LIBS=OFF",
+        "-DUSE_HDF5=OFF", "-DUSE_TESTS=OFF", "-DUSE_BINARIES=OFF",
+        "-DUSE_EXAMPLES=OFF", "-DUSE_PYALEMBIC=OFF", "-DUSE_MAYA=OFF",
+        "-DUSE_ARNOLD=OFF", "-DUSE_PRMAN=OFF",
+    ], cwd=ROOT, check=True)
+    subprocess.run(["cmake", "--build", str(BASE / "build"), "--target", "install",
+                    "--parallel", "8"], cwd=ROOT, check=True)
+
+
+if __name__ == "__main__":
+    main()