fix: ignore LOCALAPPDATA outside Windows

[Aschenbath]

When Linux (including WSL) or macOS inherits LOCALAPPDATA, Corepack currently puts its cache there instead of the platform's normal cache directory. This can send WSL downloads to a Windows filesystem and cause permission errors.

Use LOCALAPPDATA only on Windows. Preserve the existing precedence of COREPACK_HOME and XDG_CACHE_HOME, and retain the Windows home-directory fallback. Document the full selection order and add 11 unit cases covering Linux, macOS, Windows, and explicit overrides. The Linux/macOS regression cases fail before the fix.

Validation on Node.js 24.20.0 (Linux):

• Bundle and shims built successfully.
• Full Vitest suite with NOCK_ENV=replay and retries disabled: 9 files passed; 182 tests passed and 1 existing expected failure.
• TypeScript, repository-wide ESLint with --max-warnings=0, and git diff --check passed.

The platform-selection unit tests simulate process.platform. Local test setup isolates inherited proxy settings from CLI fixtures while retaining proxy access for the live npm-key check; compilation caches are stored under .git. No tests were skipped or modified for these environment adjustments.

Closes #673.

Implementation and automated validation were performed by Codex.

[aduh95]

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