feat(wasm): occ-split — lazy occ_service worker; kicad_editor drops OCC (−31%)

Move OpenCASCADE out of the merged editor image into occ_service: a separate
emscripten module (-sASYNCIFY=0, MODULARIZE, in-container -Oz finalize, 2N+8
pre-warmed pthread pool) booted lazily in a dedicated Web Worker on the first
STEP export or STEP/IGES model parse. kicad_editor.wasm ~190 MB -> 130 MB;
sessions that never touch OCC never fetch its 57 MB. STEP export works in the
browser for the first time: the unchanged desktop dialog runs EXPORTER_STEP,
whose wasm shadow suspends into globalThis.occService and the export bytes go
straight to a browser download (never entering the editor heap). STEP/IGES 3D
models parse in the worker via the oce shadow (S3D WriteCache/ReadCache wire).

- wasm/occ-service/: service CMake target (hooked from the kicad fork's
  top-level CMakeLists, wasm/editor pattern), embind entry
  (occExport/occLoadModel), wxConfig pre-js.
- wasm/stubs/{exporter_step,oce_plugin}_stub.cpp: EM_ASYNC_JS worker bridges
  (callee-shadowing; no caller #ifdefs).
- web/standalone: provider installed whenever the kicad_editor bundle boots
  (cross-face safe); ONE shared worker-boot source occ-worker.js (vite ?raw;
  the e2e stub reads the same file) — blob worker with locateFile absolutized
  against the glue URL; export download-name guard.
- deps: OCC builds with RapidJSON so its glTF/GLB writer exists — pinned to
  the vcpkg master snapshot 2025-02-26 (24b5e7a8b27f), the same code official
  KiCad consumes via vcpkg.json's opencascade[rapidjson]; rapidjson's latest
  tag (v1.1.0, 2016) is ill-formed under modern clang.
- tests: occ-export dialog e2e (lazy-fetch boundary + STEP download bytes),
  occ-probe incl. a 9-format matrix (step/stpz/brep/xao/ply/stl/glb/u3d/pdf),
  3d-viewer-models hard-asserts the worker parse; occ provider stub installed
  ambiently by the kicad fixtures.

Validated against desktop kicad-cli 10.0.4: geometric exact equality (bbox
delta 0 um, volume delta 0.0000%) for STEP/GLB/STL/BREP/STPZ across three
boards and option sweeps — with desktop OCC 7.9 vs wasm OCC 7.8; PLY/XAO/PDF
structurally equal; U3D same-size (quantizer float LSBs differ). Full kicad
e2e green on Firefox and Chromium; standalone verified end to end (lazy fetch
only on the Export click; export.step 60,628 B ISO-10303-21; loadModel 700 KB
STEP -> 569 KB scenegraph cache).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

scripts/deps/build-opencascade.sh

@@ -50,6 +50,22 @@ if [ ! -d "${OCC_DIR}" ]; then
     rm "opencascade-${OCC_VERSION}.tar.gz"
 fi
 
+# RapidJSON (header-only): required by OCC's glTF/GLB writer — without it the
+# writer compiles out (HAVE_RAPIDJSON undefined) and GLB export fails at
+# runtime with "glTF writer is unavailable". Pinned to the vcpkg master
+# snapshot (see versions.sh) — a commit archive extracts as rapidjson-<sha>,
+# so rename to the dated version dir.
+RAPIDJSON_DIR="${DEPS_ROOT}/rapidjson-${RAPIDJSON_VERSION}"
+if [ ! -d "${RAPIDJSON_DIR}" ]; then
+    log_info "Downloading RapidJSON ${RAPIDJSON_VERSION} (master snapshot ${RAPIDJSON_COMMIT:0:12})..."
+    mkdir -p "${DEPS_ROOT}"
+    cd "${DEPS_ROOT}"
+    download_file "${RAPIDJSON_URL}" "rapidjson-${RAPIDJSON_VERSION}.tar.gz"
+    tar -xzf "rapidjson-${RAPIDJSON_VERSION}.tar.gz"
+    mv "rapidjson-${RAPIDJSON_COMMIT}" "rapidjson-${RAPIDJSON_VERSION}"
+    rm "rapidjson-${RAPIDJSON_VERSION}.tar.gz"
+fi
+
 log_info "Building OpenCASCADE ${OCC_VERSION} for WASM..."
 log_warn "This is a large library and may take a while..."
 
@@ -100,7 +116,8 @@ emcmake cmake "${OCC_DIR}" \
     -DUSE_GLES2=OFF \
     -DUSE_OPENGL=OFF \
     -DUSE_D3D=OFF \
-    -DUSE_RAPIDJSON=OFF \
+    -DUSE_RAPIDJSON=ON \
+    -D3RDPARTY_RAPIDJSON_DIR="${RAPIDJSON_DIR}" \
     -DUSE_DRACO=OFF \
     -DBUILD_DOC_Overview=OFF \
     -DINSTALL_SAMPLES=OFF \