Add emscripten-wasm32 as an opt-in target platform

[Tobias-Fischer]

Summary

Adds emscripten-wasm32 support to the vinca-driven recipe pipeline, enabling ROS 2 Humble to build for WebAssembly (via the Emscripten toolchain) alongside the existing native platforms. All changes are gated by wasm32/emscripten selectors in vinca.yaml/robostack.yaml/conda_build_config.yaml — no effect on existing platform builds.

Validated end-to-end: a full ~104-package build targeting emscripten-wasm32, running a real rclcpp talker continuously in the browser via a COOP/COEP-isolated page with genuine SharedArrayBuffer-backed threads (real pthreads, not a JS shim).

Depends on RoboStack/vinca#154 (pthreads support for emscripten-wasm32) — pixi.toml currently pins the Tobias-Fischer/vinca fork branch; re-point to the merged upstream commit once that PR lands.

Key pieces

• vinca.yaml: packages_select_by_deps/skip_by_deps/remove_from_deps selectors adding a wasm32-appropriate package subset (dropping packages with no viable wasm32 host deps: fastrtps/cyclonedds RMWs, google-benchmark, gtest/gmock-heavy test infra, etc.)
• robostack.yaml: per-platform rosdep-key overrides for eigen/yaml/yaml-cpp/spdlog, since emscripten-forge doesn't publish conda packages for several of these and the vendored from-source fallback needs to be used instead
• patch/dependencies.yaml: genuinely-missing host deps added per package.xml audits (e.g. rmw_wasm_cpp needing libffi), gated to emscripten where the dep is actually missing
• patch/ros-humble-{yaml-cpp-vendor,spdlog-vendor}.emscripten.patch: fix CMAKE_PROJECT_INCLUDE propagation into each vendor's own nested ExternalProject_Add build (without it, the nested build silently loses vinca's global side-module/pthread flags)
• patch/ros-humble-rosidl-generator-py.patch: numpy 2.x header path fix (numpy/core/include → numpy/_core/include)
• patch/ros-humble-{wasm-cpp,rmw-wasm-cpp,test-wasm}.patch: patches against github.com/ros2wasm/rmw_wasm's wasm_cpp/rmw_wasm_cpp/test_wasm packages — fix a genuine embind duplicate-registration bug, drop dead SQLite3/ZLIB/BZip2 deps that broke once pthreads were enabled (emscripten-forge's prebuilt static libs for these aren't pthread-compatible, and nothing in rmw_wasm_cpp's own sources ever calls into them), and remove stale hardcoded USE_PTHREADS=0 overrides now that vinca sets it globally
• patch/ros-humble-rcutils.emscripten.patch: the actual root-cause fix for the demo running only once and then hanging forever — rcutils_steady_time_now() (which rclcpp::WallTimer actually uses) called clock_gettime(CLOCK_MONOTONIC_RAW, ...), which is silently broken in this emscripten/pthread build (the nanosecond field gets misread as the seconds field), even though plain CLOCK_MONOTONIC on the same build works correctly. Switched both rcutils_system_time_now and rcutils_steady_time_now to CLOCK_MONOTONIC under emscripten.
• vinca_pinning.yaml/conda_build_config.yaml: emscripten-forge/recipes-specific version overrides (emscripten SDK 4.0.9, python 3.13, libffi 3.4.6, graphviz 15.1.0) layered on top of the existing conda-forge-pinning-derived config, for the packages where emscripten-forge's published set doesn't match conda-forge-pinning's regular versions
• .github/workflows/testpr.yml: point the emscripten-wasm32 CI job at repo.prefix.dev/emscripten-forge-4x (repo.mamba.pm/emscripten-forge is stale/legacy, ~1/3 the package count as of 2026-09-07)

Testing

Full 104-recipe build succeeded; extracted the resulting test_wasm package into a micromamba-created emscripten-wasm32 environment, served it locally with COOP/COEP headers, and confirmed the talker demo runs continuously (55+ consecutive correctly-timed publishes) and the listener demo initializes and spins cleanly on the subscriber code path.

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