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/kernel.c

@@ -23,10 +23,31 @@
 /* Global framebuffer info, parsed from multiboot2 tags. */
 framebuffer_info_t fb_info;
 
+/**
+ * Initialize COM1 serial port for debug output.
+ * Baud rate: 115200, 8N1.
+ */
+static void serial_init(void) {
+    outb(0x3F8 + 1, 0x00);  /* Disable interrupts */
+    outb(0x3F8 + 3, 0x80);  /* Enable DLAB */
+    outb(0x3F8 + 0, 0x01);  /* Divisor low: 115200 baud */
+    outb(0x3F8 + 1, 0x00);  /* Divisor high */
+    outb(0x3F8 + 3, 0x03);  /* 8 bits, no parity, 1 stop */
+    outb(0x3F8 + 2, 0xC7);  /* Enable FIFO */
+    outb(0x3F8 + 4, 0x03);  /* RTS/DSR set */
+}
+
+static void serial_putc(char c) {
+    /* Wait for transmit buffer empty */
+    while (!(inb(0x3F8 + 5) & 0x20));
+    outb(0x3F8, c);
+}
+
 void offset_print(const char *str)
 {
     while (*str) {
-        outb(0xE9, *str);
+        outb(0xE9, *str);   /* debugcon */
+        serial_putc(*str);  /* COM1 serial */
         str++;
     }
 }
@@ -34,15 +55,30 @@ void offset_print(const char *str)
 void print_hex(uint32_t val)
 {
     const char *hex = "0123456789ABCDEF";
-    outb(0xE9, '0');
-    outb(0xE9, 'x');
+    outb(0xE9, '0'); serial_putc('0');
+    outb(0xE9, 'x'); serial_putc('x');
     for (int i = 28; i >= 0; i -= 4) {
-        outb(0xE9, hex[(val >> i) & 0xF]);
+        char c = hex[(val >> i) & 0xF];
+        outb(0xE9, c);
+        serial_putc(c);
     }
-    outb(0xE9, '\n');
+    outb(0xE9, '\n'); serial_putc('\n');
 }
 
 void kernel_main(uint32_t magic, uint32_t addr) {
+    /* Initialize serial port first so all debug output goes to COM1 too */
+    serial_init();
+
+    /* Early canary: write directly to VGA text buffer at 0xB8000.
+     * If the display is in text mode, this will show a bright magenta 'C'
+     * in the top-left corner before any subsystem is initialized. */
+    {
+        volatile uint16_t *vga = (volatile uint16_t *)0xB8000;
+        vga[0] = (uint16_t)'C' | (0x5F << 8); /* magenta on magenta = visible block */
+        vga[1] = (uint16_t)'O' | (0x5F << 8);
+        vga[2] = (uint16_t)'S' | (0x5F << 8);
+    }
+
     if (magic != MULTIBOOT2_BOOTLOADER_MAGIC) {
         offset_print("Invalid magic number: ");
         print_hex(magic);