feat: ✨ enable the 3D viewer in the WASM port (WIP)

Builds KiCad's 3D viewer for the browser (opt in with BUILD_3D_VIEWER=ON) and
wires up the legacy-GL emulation it needs.

- build-kicad-target.sh / docker/build.sh: BUILD_3D_VIEWER →
  -DKICAD_BUILD_3D_VIEWER_WASM=ON + -sLEGACY_GL_EMULATION + the GL js-library.
- wasm/shims/gl_immediate_shim.js: display-list emulation (record/replay board
  layers), fixed-function + GLU stubs, throw-guards for unsupported pnames, and
  per-context GLImmediate init for the viewer's SECOND WebGL context.
- tests/kicad/3d-viewer.spec.ts + utils/pcbnew-ready.ts: open View → 3D Viewer.
- bumps the kicad submodule (EMSCRIPTEN-guarded GL changes).

WIP: the viewer window + UI render and geometry draws (glError=0x0), but the
board is not yet visibly rendered — GLImmediate's FFP shader program isn't
linked on the viewer's 2nd WebGL context. Full status + next steps in
features/feat/add-3d-view/notes.md.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docker/build.sh

@@ -188,6 +188,7 @@ compile_app() {
     # emsdk_env.sh, so the build shell would lack emcc/embuilder on PATH. Setting
     # EMSDK lets scripts/common/env.sh source /emsdk/emsdk_env.sh and activate the toolchain.
     docker compose -f docker/docker-compose.yml exec -e EMSDK=/emsdk \
+        -e BUILD_3D_VIEWER="${BUILD_3D_VIEWER:-OFF}" \
         kicad-wasm-builder \
         "/workspace/scripts/kicad/build-${app}.sh" "${ARGS[@]}"