feat(sfs): Implemented Sovereign Filesystem (SFS)

- Implemented SFS Driver (core/fs/sfs.nim):
  - Mount logic (Sector 0 Superblock check).
  - List logic (Sector 1 Directory table).
- Implemented Userland Formatter (nipbox.nim):
  - 'mkfs' command to write SFS1 Superblock.
- Fixed 'virtio_block' logic:
  - Corrected Descriptor flags (VRING_DESC_F_WRITE for Read Buffers).
- Fixed Async/Sync Conflict in 'libc_shim':
  - Added 'nexus_yield()' to block syscalls to prevent stack corruption before kernel processing.
- Integrated SFS into Kernel startup.

nipbox.nim

@@ -81,9 +81,10 @@ proc print_raw(s: string) =
   if s.len > 0:
     discard write(1, unsafeAddr s[0], csize_t(s.len))
 
-# Helper: Swap Bytes 16
-proc swap16(x: uint16): uint16 =
-  return (x shl 8) or (x shr 8)
+# Forward declarations for functions used before their definition
+proc parseIntSimple(s: string): uint64
+proc toHexChar(b: byte): char
+proc do_mkfs()
 
 # Calculate Checksum (Standard Internet Checksum)
 proc calc_checksum(buf: cptr, len: int): uint16 =
@@ -346,8 +347,44 @@ proc main() =
     elif cmd == "ls": do_ls()
     elif cmd == "exec": do_exec(arg)
     elif cmd == "dd": do_dd(arg)
+    elif cmd == "mkfs": do_mkfs()
     elif cmd == "help": do_help()
     else: print("Unknown command: " & cmd)
 
+proc do_mkfs() =
+  print("[mkfs] Formatting disk as Sovereign Filesystem (SFS v1)...")
+
+  # 1. Superblock (Sector 0)
+  var sb: array[512, byte]
+  # Magic: S (0x53), F (0x46), S (0x53), 1 (0x31) -> Little Endian? String is byte order.
+  sb[0] = 0x53
+  sb[1] = 0x46
+  sb[2] = 0x53
+  sb[3] = 0x31
+  # Disk Size (u64 at offset 4) - 32MB = 33554432 = 0x02000000
+  # Little Endian
+  sb[4] = 0x00
+  sb[5] = 0x00
+  sb[6] = 0x00
+  sb[7] = 0x02
+  sb[8] = 0x00
+  sb[9] = 0x00
+  sb[10] = 0x00
+  sb[11] = 0x00
+
+  # Root Dir Sector (u64 at offset 12) -> 1
+  sb[12] = 0x01
+
+  nexus_blk_write(0, addr sb[0], 512)
+  print("[mkfs] Superblock written.")
+
+  # 2. Directory Table (Sector 1) - Zero it
+  var zero: array[512, byte] # Implicitly zeroed? In Nim, yes if global/stack? Better be safe.
+  for i in 0 ..< 512: zero[i] = 0
+  nexus_blk_write(1, addr zero[0], 512)
+  print("[mkfs] Directory Table initialized.")
+
+  print("[mkfs] Format Complete. The Ledger is structured.")
+
 when isMainModule:
   main()