fix(core): resolve docs.mjs package dir with fileURLToPath so component docs work on Windows

[arham766]

Summary

docs.mjs resolved its package directory with new URL(import.meta.url).pathname, which returns /D:/... for drive-letter paths on Windows. srcDir then pointed at a nonexistent D:\D:\... path, so --list silently printed nothing (the existsSync guard returned an empty catalog) and single-component lookup crashed with ENOENT. Switching to fileURLToPath fixes both and makes directory resolution symmetric with the existing pathToFileURL use in loadDoc().

Adds subprocess smoke tests that spawn the script from a neutral cwd (os.tmpdir()), so resolution can only come from the module URL. The test lives in src/ (not next to docs.mjs) because the vitest include glob only picks up packages/**/src/**; noted in the file header. No overlap with #3234 (CLI test paths) or #3305 (core build script).

Test plan

• npx vitest run packages/core/src/docs-script.test.mjs passes on Windows (2/2); the fix is a no-op on POSIX where fileURLToPath also correctly decodes percent-encoded paths.
• Manual on Windows: node packages/core/docs.mjs --list now prints the full catalog; node packages/core/docs.mjs Button renders full docs. Before the fix, the former printed nothing and the latter exited with ENOENT.
• Full monorepo suite run on Windows and diffed against a main-branch baseline: no new failures introduced.
• node scripts/check-changesets.mjs passes.

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[arham766]

@cixzhang gentle ping when you have a minute. Small fix, unblocks Windows users of the core docs script (both --list and single-component lookup are broken there today).