fix(memfs): resolve relative streams from virtual cwd

[xianjianlf2]

Summary

• resolve Superblock relative paths against the volume's process-like cwd instead of the host process cwd
• give the default memfs export and memfs() helper a virtual-root cwd by default
• add regression coverage for relative createWriteStream paths on memfs() and the default fs export

Fixes #900.

Tests

• yarn build
• yarn jest packages/memfs/src/tests/memfs.process.test.ts --runInBand
• yarn jest packages/fs-node/src/tests/volume.process.test.ts --runInBand
• yarn prettier --check packages/fs-core/src/Superblock.ts packages/fs-node/src/tests/volume.process.test.ts packages/memfs/src/tests/memfs.process.test.ts packages/memfs/src/index.ts
• node runtime check for default fs.createWriteStream('relative') and memfs().fs.createWriteStream('relative')

[Copilot]

Pull request overview

This PR fixes relative-path stream operations in memfs by ensuring path resolution uses the virtual filesystem’s own process-like cwd() rather than the host Node.js process working directory, matching expected fs semantics and addressing #900.

Changes:

• Resolve Superblock relative paths against the volume’s configured process.cwd() for core filesystem operations.
• Set the default exported memfs instance (and memfs() helper default) to use a virtual-root cwd ('/') so relative paths land inside the volume.
• Add regression tests covering relative createWriteStream() behavior for both memfs() instances and the default exported fs.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
packages/memfs/src/index.ts	Introduces a process-like wrapper to provide a virtual cwd and updates default volume initialization to avoid host-cwd resolution.
packages/memfs/src/tests/memfs.process.test.ts	Adds regression coverage for relative createWriteStream() paths on memfs() and the default exported fs.
packages/fs-node/src/tests/volume.process.test.ts	Verifies that relative writes resolve against a custom process.cwd() when provided to a Volume.
packages/fs-core/src/Superblock.ts	Routes filename-to-steps resolution through the volume’s process.cwd() so relative paths resolve within the virtual FS context.