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.c

@@ -12,6 +12,7 @@
 #include "env.h"
 #include "port_io.h"
 #include "vga.h"
+#include "vfs.h"
 #include "keyboard.h"
 #include "cpio.h"
 #include "paging.h"
@@ -281,6 +282,30 @@ static int32_t sys_setenv(registers_t *regs) {
     return env_set(&cur->env, key, value);
 }
 
+/**
+ * Handle SYS_READDIR: read a directory entry.
+ * EBX = path, ECX = index, EDX = name buffer (128 bytes min).
+ * Returns entry type (VFS_FILE=1, VFS_DIRECTORY=2, ...) on success, -1 at end.
+ */
+static int32_t sys_readdir(registers_t *regs) {
+    const char *path = (const char *)regs->ebx;
+    uint32_t idx = regs->ecx;
+    char *name_buf = (char *)regs->edx;
+
+    vfs_dirent_t entry;
+    if (vfs_readdir(path, idx, &entry) != 0) {
+        return -1;
+    }
+
+    /* Copy entry name to user buffer */
+    uint32_t len = strlen(entry.name);
+    if (len >= 128) len = 127;
+    memcpy(name_buf, entry.name, len);
+    name_buf[len] = '\0';
+
+    return (int32_t)entry.type;
+}
+
 /** System call dispatch table. */
 typedef int32_t (*syscall_fn)(registers_t *);
 static syscall_fn syscall_table[NUM_SYSCALLS] = {
@@ -294,6 +319,7 @@ static syscall_fn syscall_table[NUM_SYSCALLS] = {
     [SYS_EXEC]    = sys_exec,
     [SYS_GETENV]  = sys_getenv,
     [SYS_SETENV]  = sys_setenv,
+    [SYS_READDIR] = sys_readdir,
 };
 
 void syscall_handler(registers_t *regs) {