OCCT dual-version harden: Handle aliases, failure messages, warning cleanup

[trailcode]

Summary

• Harden shared src/ for desktop OCCT 8 vs WASM OCCT 7.9.3: standard_failure_message(), *_ptr aliases instead of Handle(T), agent conventions for both.
• Clear native/WASM compiler warnings (nodiscard clear_tmps, Standard_Real, doc-link wchar narrowing, ImGui dock enum OR under emscripten).

Closes #196

Test plan

• Native Debug rebuild EzyCad_lib / EzyCad_tests (listed C4834 / C4996 / C4244 gone)
• WASM build-em-793 ninja link succeeds; dock enum warning addressed
• CI Windows MSVC green
• Spot-check fillet/chamfer/revolve error paths still surface OCCT messages
• Spot-check Esc cancel still two-stage (tmp tool then parent mode)

Notes

• Temporary agent note agents/conventions/occt-wasm-dual-version.md should be retired when wasm on OCCT 8 is supported.
• Prefer agents/conventions/occt-handles.md for new handle types (add to utl_types.h).

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