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

apps/ls/ls.c

@@ -0,0 +1,36 @@
+/**
+ * @file ls.c
+ * @brief List directory contents.
+ *
+ * Lists entries in the current working directory (from the CWD
+ * environment variable). Uses the SYS_READDIR syscall to enumerate
+ * directory entries.
+ */
+
+#include "syscalls.h"
+
+int main(void) {
+    /* Get the current working directory */
+    char cwd[128];
+    if (getenv("CWD", cwd, sizeof(cwd)) < 0) {
+        /* Default to root if CWD not set */
+        cwd[0] = '/';
+        cwd[1] = '\0';
+    }
+
+    char name[128];
+    uint32_t idx = 0;
+    int32_t type;
+
+    while ((type = readdir(cwd, idx, name)) >= 0) {
+        puts(name);
+        if (type == 2) {
+            /* Directory: append / indicator */
+            putchar('/');
+        }
+        putchar('\n');
+        idx++;
+    }
+
+    return 0;
+}