feat: STEP write under wasm (pyodide 0.29.4 / native wasm-EH)

[Krande]

Summary

backend.write_step (STEPCAFControl_Writer via OCCT XCAF) fatally aborted the pyodide module with RuntimeError: unreachable / bare Aborted(). This fixes it, and migrates the wasm toolchain to pyodide 0.29.4 / emscripten 4.0.9 / Python 3.13 with native WebAssembly exception handling (required for the fix).

Root cause (two parts)

1. step_writer.cpp / helpers.cpp were never compiled into the wasm side module. The BUILD_WASM branch of CMakeLists.txt still carried the stale "OCCT not available in wasm" source list (Color.cpp + Models.cpp only). So write_shapes_to_step was emitted as an unresolved env import whose function-table slot is a trap stub — the first call trapped with unreachable. GLB / STEP-read / make_box worked only because they are self-contained in cad_py_wrap.cpp.
2. The OCAF document used "XmlOcaf", whose driver isn't registered under OCCT_NO_PLUGINS (TKXml* isn't even linked for wasm). Switched to BinXCAFDrivers::DefineFormat + a "BinXCAF" document (TKBinXCAF is linked). write_shapes_to_step now also translates Standard_Failure into a std::runtime_error so nanobind surfaces a Python exception instead of std::terminate.

Toolchain migration (pyodide 0.29.4 / wasm-EH)

The old -fexceptions JS-trampoline EH model fatally trapped in STEPCAFControl_Writer; the fix needs native wasm exception handling:

• OCCT + adacpp + nanobind all compile with -fwasm-exceptions (+ -sSUPPORT_LONGJMP=wasm).
• OCC_CONVERT_SIGNALS dropped on wasm — setjmp inside a catch is invalid under wasm-EH and made emscripten emit invalid wasm (br_table: label arity inconsistent CompileError).
• wheel tag cp313 / pyodide_2025_0_wasm32; Python_SOABI = cpython-313-wasm32-emscripten.
• xbuildenv path resolved dynamically via pyodide config get python_include_dir instead of a machine-specific content-hash path (CI-portable).
• native test env + linux preset bumped to Python 3.13 to match feature.common.

Verification

• WASM (node-pyodide): SAT → {glb, obj, stl, xml, step} all ✅ (step = 6422 bytes) and FEM bake (manifest + mesh + edges + elements + 3 result fields) ✅.
• WASM, from-scratch build with the dynamic xbuildenv path (full OCCT cross-compile) — wheel builds and all targets pass, validating the CI flow.
• Native: full suite 62 passed, including test_cad_write_step / test_basic_write_step (the BinXCAF path) — works outside wasm too.

CI note

This PR touches cmake/wasm_occt.cmake / pixi.toml / CMakePresets.json, so merging triggers publish-occt-wasm-base to rebuild the OCCT-wasm base image with the new EH model (em4.0.9, -fwasm-exceptions, no OCC_CONVERT_SIGNALS). ci-wasm-tests / publish-wasm-base consume the base via :latest; if they run before the rebuilt base finishes they may pick up the stale (em3.1.58) base and need a re-run once the base publishes. The fallback (build OCCT from source) still produces a correct result.

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