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/libc/syscalls.h

@@ -23,6 +23,7 @@ typedef int int32_t;
 #define SYS_EXEC    7
 #define SYS_GETENV  8
 #define SYS_SETENV  9
+#define SYS_READDIR 10
 
 static inline int32_t syscall0(int num) {
     int32_t ret;
@@ -91,6 +92,17 @@ static inline int32_t setenv(const char *key, const char *value) {
     return syscall2(SYS_SETENV, (uint32_t)key, (uint32_t)value);
 }
 
+/**
+ * Read a directory entry.
+ * @param path  Directory path.
+ * @param idx   Entry index (0-based).
+ * @param name  Buffer for entry name (128 bytes min).
+ * @return Entry type (1=file, 2=dir) on success, -1 at end.
+ */
+static inline int32_t readdir(const char *path, uint32_t idx, char *name) {
+    return syscall3(SYS_READDIR, (uint32_t)path, idx, (uint32_t)name);
+}
+
 /* Basic string operations for user-space */
 static inline uint32_t strlen(const char *s) {
     uint32_t len = 0;