Support PEP 783 pyemscripten_*_wasm32 wheel platform tag

[messense]

The Emscripten platform-tag branch in get_platform_tag now resolves the wheel tag through a priority cascade so maturin produces installable wheels across the full Pyodide release spectrum:

1. pyemscripten_<year>_<patch>_wasm32 (PEP 783, Pyodide >= 0.30 / Python 3.14+), driven by MATURIN_PYEMSCRIPTEN_PLATFORM_VERSION / PYEMSCRIPTEN_PLATFORM_VERSION or pyodide config get pyemscripten_platform_version.
2. pyodide_<year>_<patch>_wasm32 (Pyodide 0.28 / 0.29), driven by MATURIN_PYODIDE_ABI_VERSION / PYODIDE_ABI_VERSION or pyodide config get pyodide_abi_version.
3. Legacy emscripten_<emcc-version>_wasm32 (Pyodide <= 0.27), still driven by MATURIN_EMSCRIPTEN_VERSION / emcc -dumpversion. A warning notes that wheels in this format are not installable on PEP 783-compliant runtimes.

The cascade is implemented as a pure function over an EmscriptenVersionInputs struct so the per-Pyodide-version behaviour can be covered deterministically by unit tests, and version segments are validated against the PEP 783 [0-9]+_[0-9]+ regex so malformed inputs fail loudly instead of producing a tag that no installer accepts.

The generated GitHub Actions workflow (generate-ci) now also exports PYEMSCRIPTEN_PLATFORM_VERSION and PYODIDE_ABI_VERSION from pyodide config get, defaulting to empty so older Pyodide releases keep producing the legacy tag.

For real-Pyodide coverage, noxfile.py learns to parse pyodide-lock.json (info.platform, info.abi_version, and a future info.pyemscripten_platform_version) into the matching env vars plus an EXPECTED_PLATFORM_TAG that test-emscripten asserts against the produced wheel filename. The CI test-emscripten job is now a matrix over Pyodide 0.28 and 0.29, and tests/emscripten_runner.js was updated to match all three Emscripten tag families when locating the built wheel.