feat: 🎸 add file birthTime support

[streamich]

No description provided.

[Copilot]

Pull request overview

This PR adds proper birthtime support by introducing a dedicated node creation timestamp (btime) in fs-core and wiring it through to fs-node Stats.birthtime*, with new tests to validate stability across common file operations.

Changes:

• Added Node.btime (birth/creation time) and included it in Node.toJSON().
• Updated Stats.build() to report birthtime, birthtimeMs, and birthtimeNs from Node.btime instead of ctime.
• Added test coverage in both fs-core (node-level invariants) and fs-node (stat-level behavior across operations).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
packages/fs-node/src/Stats.ts	Maps Node.btime into Stats.birthtime* (including bigint variants).
packages/fs-node/src/tests/birthtime.test.ts	Adds integration tests ensuring birthtime stability across writes/metadata changes and bigint stats.
packages/fs-core/src/Node.ts	Introduces btime on nodes and serializes it via toJSON().
packages/fs-core/src/tests/Node.test.ts	Adds unit tests asserting btime initialization and immutability across node mutations.

Suppressed comments (1)

packages/fs-core/src/Node.ts:363

• Node.toJSON() currently reads file contents via getString(), which updates atime (and does so twice because getString() calls getBuffer(), which also updates atime). Serialization shouldn’t have side effects on timestamps; with btime now explicitly modeled, this kind of hidden mutation becomes more surprising for snapshot/restore-style uses.

      btime: this.btime.getTime(),
      perm: this.perm,
      mode: this.mode,
      nlink: this.nlink,
      symlink: this.symlink,