Fix UTM display: remove required framebuffer tag, add serial output, ls app

Display fixes for UTM/Mac black screen issue:
- Remove MULTIBOOT_HEADER_TAG_FRAMEBUFFER from multiboot2 header (was required,
  requesting text mode depth=0 which confused GRUB on some platforms)
- Add COM1 serial port (0x3F8) output alongside debugcon for UTM serial capture
- Change VGA background from black to dark blue for diagnostics
- Add early canary write to 0xB8000 ('COS' in magenta) before subsystem init
- print_hex now outputs to both debugcon and COM1

New ls command and SYS_READDIR syscall:
- SYS_READDIR (10): reads directory entries via VFS
- VFS root listing: vfs_readdir handles '/' by iterating mount table
- apps/ls: lists CWD contents, appends '/' for directories
- apps/libc/syscalls.h: readdir() wrapper

src/syscall.h

@@ -24,9 +24,10 @@
 #define SYS_EXEC    7   /**< Execute a program. path=EBX, argv=ECX. */
 #define SYS_GETENV  8   /**< Get environment variable. key=EBX, buf=ECX, bufsize=EDX. */
 #define SYS_SETENV  9   /**< Set environment variable. key=EBX, value=ECX. */
+#define SYS_READDIR 10  /**< Read directory entry. path=EBX, idx=ECX, buf=EDX. Returns type or -1. */
 
 /** Total number of system calls. */
-#define NUM_SYSCALLS 10
+#define NUM_SYSCALLS 11
 
 /**
  * Initialize the system call handler.