New src/shared/xattr.{c,h}: capture user.* + POSIX ACL xattrs, transmit a bounded per-file block, re-apply fd-relative. security.*/trusted.*/other system.* never transmitted/applied (receiver re-validates). Bounds: name<=255 value<=1MiB count <=256 total<=4MiB. --fake-super records uid:gid:mode:mtime in reserved user.fastsync.stat (receiver-only). PROTOCOL_VERSION 2.12.0->2.13.0. Review fixes: reserved key not forwardable, link/hardlink copy-fallback preserves xattrs, no const-param mutation, per-file warning dedup.
