Implement mkfs.fat32 filesystem formatter app (AI)

Create a user-space utility that formats block devices with FAT32.
Writes all required on-disk structures sequentially:

- Boot sector (BPB) with BIOS Parameter Block at sector 0
- FSInfo sector at sector 1 with free cluster count/hint
- Backup boot sector at sector 6, backup FSInfo at sector 7
- Two FAT tables with entries 0-2 initialized (media marker,
  EOC markers, root directory cluster EOC)
- Root directory cluster with volume label entry

Geometry calculation:
- Sectors per cluster chosen based on volume size (1-32)
- FAT size computed using Microsoft's formula
- Supports volumes from 128 sectors (~64K) upward

Usage: mkfs.fat32 hdd1mbr1 14336 [LABEL]
The total sector count is required since there is no stat() syscall
yet. Users can find it via 'diskpart <dev> list'.

Also checked off diskpart in README (committed previously).

build.log

@@ -13,6 +13,13 @@ Building app: hello-world
   Built: /workspaces/claude-os/build/apps_bin/hello-world (49 bytes)
 Building app: ls
   Built: /workspaces/claude-os/build/apps_bin/ls (250 bytes)
+Building app: mkfs.fat32
+/workspaces/claude-os/apps/mkfs.fat32/mkfs.fat32.c:56:13: warning: unused function 'print_hex' [-Wunused-function]
+   56 | static void print_hex(uint32_t val) {
+      |             ^~~~~~~~~
+1 warning generated.
+/usr/bin/ld: warning: /workspaces/claude-os/build/apps_bin/mkfs.fat32.elf has a LOAD segment with RWX permissions
+  Built: /workspaces/claude-os/build/apps_bin/mkfs.fat32 (5121 bytes)
 Building app: mount
   Built: /workspaces/claude-os/build/apps_bin/mount (992 bytes)
 Building app: sh
@@ -23,7 +30,7 @@ Building app: sh
   Built: /workspaces/claude-os/build/apps_bin/sh (3428 bytes)
 [  3%] Built target apps
 [  6%] Generating CPIO initial ramdisk
-Generated initrd: 15040 bytes
+Generated initrd: 20288 bytes
 [  6%] Built target initrd
 [ 96%] Built target kernel
 [100%] Generating bootable ISO image
@@ -33,14 +40,14 @@ Drive current: -outdev 'stdio:/workspaces/claude-os/release/claude-os.iso'
 Media current: stdio file, overwriteable
 Media status : is blank
 Media summary: 0 sessions, 0 data blocks, 0 data,  126g free
-Added to ISO image: directory '/'='/tmp/grub.MJcmop'
+Added to ISO image: directory '/'='/tmp/grub.bEiDnH'
 xorriso : UPDATE :     581 files added in 1 seconds
 Added to ISO image: directory '/'='/workspaces/claude-os/build/isodir'
 xorriso : UPDATE :     586 files added in 1 seconds
 xorriso : NOTE : Copying to System Area: 512 bytes from file '/usr/lib/grub/i386-pc/boot_hybrid.img'
-xorriso : UPDATE :  96.99% done
-ISO image produced: 5922 sectors
-Written to medium : 5922 sectors at LBA 0
+xorriso : UPDATE : Thank you for being patient. Working since 0 seconds.
+ISO image produced: 5924 sectors
+Written to medium : 5924 sectors at LBA 0
 Writing to 'stdio:/workspaces/claude-os/release/claude-os.iso' completed successfully.
 
 [100%] Built target iso