Since #2609, the pyodide platform required the 'pyodide-build' frontend,
but a global build-frontend setting (TOML or CIBW_BUILD_FRONTEND)
overrides the platform default, so previously-working configs like
build-frontend = "build" failed with a ConfigurationError. Even
"default" failed, since it was mapped to "build" before the check.
Resolve "default" to the platform default, and warn and use
pyodide-build when another frontend is set on pyodide, matching
pre-#2609 behavior where the frontend name only affected verbosity
flags.
Assisted-by: ClaudeCode:claude-opus-4.8