feat: default to Yarn Modern on fresh installs

[Aschenbath]

A fresh Corepack installation now offers Yarn Modern for commands such as yarn create, using the bundled Yarn 4.18.0 release when latest lookups are disabled and the stable Yarn release index otherwise.

• Keep the weekly version sync on Yarn Modern and document the default; project pins and explicitly activated Yarn Classic releases remain supported.
• Resolve Yarn Modern through @yarnpkg/cli-dist when COREPACK_NPM_REGISTRY is configured. Verify the downloaded tarball separately from the extracted binary so registry integrity hashes are not treated as binary hashes.
• Cover fresh installs, stable release selection, custom registries, offline reuse, and Classic compatibility.

Validation:

• ./node_modules/.bin/tsc --noEmit
• ./node_modules/.bin/eslint --max-warnings=0
• Vitest with NOCK_ENV=replay, using this checkout's node_modules; proxy variables cleared for the replay/custom-registry suites, and the live npm key-store check run separately with the environment proxy. The two offline cases were rerun with the inherited NODE_USE_ENV_PROXY unset because their dummy proxy values otherwise fail during Node startup.

Results: 180 tests passed across all 9 test files, plus 1 existing expected-failure case (full replay run, offline-case rerun, and the separate npm key-store check). The 42 focused default/registry/integrity tests also passed.

Closes #812

[aduh95]

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