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>

tests/fixtures/demo/demo.kicad_prl

@@ -0,0 +1,105 @@
+{
+  "board": {
+    "active_layer": 0,
+    "active_layer_preset": "",
+    "auto_track_width": true,
+    "hidden_netclasses": [],
+    "hidden_nets": [],
+    "high_contrast_mode": 0,
+    "net_color_mode": 1,
+    "opacity": {
+      "images": 0.6,
+      "pads": 1.0,
+      "shapes": 1.0,
+      "tracks": 1.0,
+      "vias": 1.0,
+      "zones": 0.6
+    },
+    "prototype_zone_fills": false,
+    "selection_filter": {
+      "dimensions": true,
+      "footprints": true,
+      "graphics": true,
+      "keepouts": true,
+      "lockedItems": false,
+      "otherItems": true,
+      "pads": true,
+      "text": true,
+      "tracks": true,
+      "vias": true,
+      "zones": true
+    },
+    "visible_items": [
+      "vias",
+      "footprint_text",
+      "footprint_anchors",
+      "ratsnest",
+      "grid",
+      "footprints_front",
+      "footprints_back",
+      "footprint_values",
+      "footprint_references",
+      "tracks",
+      "drc_errors",
+      "drawing_sheet",
+      "bitmaps",
+      "pads",
+      "zones",
+      "drc_warnings",
+      "drc_exclusions",
+      "locked_item_shadows",
+      "conflict_shadows",
+      "shapes",
+      "board_outline_area",
+      "ly_points"
+    ],
+    "visible_layers": "ffffffff_ffffffff_ffffffff_ffffffff",
+    "zone_display_mode": 0
+  },
+  "git": {
+    "integration_disabled": false,
+    "repo_type": "",
+    "repo_username": "",
+    "ssh_key": ""
+  },
+  "meta": {
+    "filename": "demo.kicad_prl",
+    "version": 5
+  },
+  "net_inspector_panel": {
+    "col_hidden": [],
+    "col_order": [],
+    "col_widths": [],
+    "custom_group_rules": [],
+    "expanded_rows": [],
+    "filter_by_net_name": true,
+    "filter_by_netclass": true,
+    "filter_text": "",
+    "group_by_constraint": false,
+    "group_by_netclass": false,
+    "show_time_domain_details": false,
+    "show_unconnected_nets": false,
+    "show_zero_pad_nets": false,
+    "sort_ascending": true,
+    "sorting_column": -1
+  },
+  "open_jobsets": [],
+  "project": {
+    "files": []
+  },
+  "schematic": {
+    "hierarchy_collapsed": [],
+    "selection_filter": {
+      "graphics": true,
+      "images": true,
+      "labels": true,
+      "lockedItems": false,
+      "otherItems": true,
+      "pins": true,
+      "ruleAreas": true,
+      "symbols": true,
+      "text": true,
+      "wires": true
+    }
+  }
+}