squashfs: expose metadata via StatT (alternative to #395)

[luthermonson]

Refs #301. Alternative design to #395 — same goal, restructured around a StatT struct to match the pattern used by the ext4 (#393) and iso9660 (#394) siblings.

What changed vs #395

#395 added two more accessor methods (Inode(), InodeType()) on top of the existing UID()/GID()/Xattrs()/Readlink() surface reachable through squashfs.FileStat. That kept squashfs internally consistent but left it stylistically out of step with the other filesystems answering #301, which all expose a StatT struct.

This PR goes the other way: it converts the squashfs Sys() surface to a StatT struct too, accepting the breaking change to get one consistent shape across the package.

if st, ok := fi.Sys().(*squashfs.StatT); ok {
    fmt.Println(st.Inode, st.InodeType, st.UID, st.LinkTarget)
}

Fields

Field	Source
UID uint32	directory entry
GID uint32	directory entry
Inode uint32	inode index
InodeType string	one of 14 documented values, or "unknown"
Xattrs map[string]string	directory entry
LinkTarget string	symlink target, empty for non-symlinks

Breaking changes

Removed from the public surface:

• type FileStat = *directoryEntry alias
• (FileStat).UID() uint32
• (FileStat).GID() uint32
• (FileStat).Xattrs() map[string]string

External callers migrate:

// before
if st, ok := fi.Sys().(squashfs.FileStat); ok {
    fmt.Println(st.UID(), st.Xattrs())
}

// after
if st, ok := fi.Sys().(*squashfs.StatT); ok {
    fmt.Println(st.UID, st.Xattrs)
}

Readlink() and Open() remain as methods on the now-unexported directoryEntry for internal use, but are no longer reachable externally. Symlink targets are exposed via StatT.LinkTarget; opening files continues through FileSystem.OpenFile.

Tests

• TestSquashfsStatT covers the new struct: non-zero Inode, valid InodeType, directory entries report a directory type, empty LinkTarget for regular files, also reachable from OpenFile().Stat().
• TestDirectoryEntry (internal) updated to assert on *StatT.
• TestSquashfsOpen, TestSquashfsCheckListing, TestSquashfsReadDirXattr updated to the new shape.

Which to merge

This is meant as a comparison PR — if you prefer the consistency across packages, merge this and close #395; if you prefer keeping squashfs's existing accessor surface, merge #395 and close this.

[deitch]

This is nice and clean. Merging this, closing #395