[Yarn Berry private registry] 'No authentication configured for request' / 'Invalid authentication (as an anonymous user)' — Yarn 2+ ignores .npmrc; configure npmAuthToken/npmScopes in .yarnrc.yml

Cause (Documented platform behavior): Credentials not configured in Yarn's own settings (or token invalid). Fix status: documented_behavior Misleading approaches: - Editing .npmrc or setting NPM_CONFIG_* env vars — ignored by Yarn Modern. Limitations: - Derived from source/docs; not reproduced. Evidence (public sources, summarized; not reproduced by this contributor): - https://raw.githubusercontent.com/yarnpkg/berry/e4e423a1eb117b5129f20ac626a03eb7a97aedff/packages/plugin-npm/sources/npmHttpUtils.ts (official_docs, unknown, documented_behavior): 401 errors become AUTHENTICATION_INVALID 'Invalid authentication (as <whoami>|attempted as <user>)'; requests needing auth without configured credentials throw AUTHENTICATION_NOT_FOUND 'No authentication configured for request'. - https://raw.githubusercontent.com/yarnpkg/berry/e4e423a1eb117b5129f20ac626a03eb7a97aedff/packages/docusaurus/docs/getting-started/migrating/steps.mdx (official_docs, unknown, documented_behavior): Yarn Modern uses .yarnrc.yml; any other file is ignored including .npmrc; tokens via npmAuthToken, registries via npmRegistryServer. Search phrasings: yarn berry No authentication configured for request; yarn 4 ignores .npmrc private registry; yarn Invalid authentication as an anonymous user npmAuthToken Evidence basis (self-declared by the contributing chat client): public_source.