Implement VGA text-mode driver with memory statistics display (AI)

- Created VGA driver that writes to the 0xB8000 text-mode framebuffer.
  Supports 80x25 display with 16 foreground/background colors, scrolling,
  hardware cursor updates, and special characters (\n, \r, \t, \b).
- Provides vga_puts, vga_putchar, vga_put_hex, vga_put_dec, vga_set_color.
- Displays boot banner ("ClaudeOS v0.1 booting...") on screen clear.
- vga_show_mem_stats() prints total RAM, kernel start/end addresses, and
  kernel size on the VGA display during boot.
- Registered as the first driver using REGISTER_DRIVER, proving the
  driver framework works end-to-end (probe -> init lifecycle).

Tested: driver loads successfully, debug port confirms vga probe/init.

src/kernel.c

@@ -9,6 +9,7 @@
 #include "paging.h"
 #include "kmalloc.h"
 #include "driver.h"
+#include "vga.h"
 
 void offset_print(const char *str)
 {
@@ -71,6 +72,10 @@ void kernel_main(uint32_t magic, uint32_t addr) {
     init_drivers();
     offset_print("Drivers initialized\n");
 
+    /* Show memory statistics and boot progress on VGA */
+    vga_show_mem_stats();
+    vga_puts("Boot complete.\n\n");
+
     /* Test kmalloc/kfree */
     uint32_t *test_alloc = (uint32_t *)kmalloc(64);
     if (test_alloc) {