feat: add fragment table, file dedup, xattrs, and CloneInode

Fragment table: file tails smaller than blockSize are packed into shared
fragment blocks, significantly reducing image size for many small files.
Fragment table written as compressed metadata with indirect pointer table.

File deduplication: CloneInode(newPath, existingPath) creates entries that
share underlying data for any inode type. AddFS auto-detects hard links
via dev+ino on Unix platforms. Cloned files promoted to XFileType.

Extended attributes: SetXattr(path, name, value) API for programmatic
use. AddFS reads xattrs via XattrFS interface or platform syscalls.
Xattr table serialized per squashfs spec with k-v metadata, ID table,
and header. Inodes with xattrs auto-promoted to extended types.

Also: directory entry type field now uses Basic() for extended types,
all extended inode types (XFile, XSymlink, XDev, XFifo, XSocket)
serialized in writeInode, XDirType xattrIdx no longer hardcoded.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: MagicalTux

README.md

@@ -194,9 +194,4 @@ go install -tags "xz zstd" github.com/KarpelesLab/squashfs/cmd/sqfs@latest
 
 # Limitations
 
-The writer does not currently support:
-
-* Fragment tables (tail-end packing of small files)
-* NFS export tables
-* File deduplication
-* Extended attributes (xattrs)
+* NFS export tables are not implemented

hardlink.go

@@ -0,0 +1,7 @@
+package squashfs
+
+// devIno uniquely identifies a file on a device for hard link detection.
+type devIno struct {
+	dev uint64
+	ino uint64
+}

hardlink_other.go

@@ -0,0 +1,8 @@
+//go:build !unix
+
+package squashfs
+
+// getDevIno is a no-op on non-Unix platforms.
+func getDevIno(sys any) (devIno, bool) {
+	return devIno{}, false
+}

hardlink_unix.go

@@ -0,0 +1,17 @@
+//go:build unix
+
+package squashfs
+
+import "syscall"
+
+// getDevIno extracts the device and inode numbers from a FileInfo.Sys() value.
+func getDevIno(sys any) (devIno, bool) {
+	if sys == nil {
+		return devIno{}, false
+	}
+	st, ok := sys.(*syscall.Stat_t)
+	if !ok {
+		return devIno{}, false
+	}
+	return devIno{dev: uint64(st.Dev), ino: uint64(st.Ino)}, true
+}