RFC 0004: Epic / Tracking issue for extended file attributes

[rmanibus]

Context

Track completion and rollout for RFC 0004 (Extended File Attributes).

RFC: rfcs/0004-extended-file-attributes.md

Goal

Persist rich filesystem metadata in snapshots and restores where available, while keeping behavior portable and safe when metadata is unavailable.

Scope

• Capture xattrs and relevant POSIX metadata during backup for supported sources/filesystems.
• Include metadata in snapshot/filemeta model and scan engine behavior.
• Provide CLI controls for metadata collection (-skip-mode, -skip-flags, -skip-xattrs, -xattr-namespaces).
• Keep cross-platform compatibility with graceful fallback/stub behavior.

Implementation notes

Current implementation branch: feat/extended-file-attributes (PR #78).

Acceptance Criteria

• Metadata fields are represented in core models and flow through backup scan paths.
• Local source metadata collection supports platform-specific behavior with tests.
• CLI flags and docs describe behavior and tradeoffs.
• Snapshot creation remains stable when metadata is missing/inaccessible.
• Tests cover parsing, scan behavior, and platform stubs.

Linked PR

• feat: extended file attributes (RFC 0004) #78 feat: extended file attributes

[rmanibus]

Implementation PR: #78

[rmanibus]

Related RFC PR: #65