Add VFS subsystem and initrd filesystem driver (AI)

VFS subsystem (vfs.c/h):
- Mount table with up to 16 mount points, longest-prefix path matching.
- File descriptor table (256 entries, fds 0-2 reserved for std streams).
- Path resolution walks mount table then delegates to filesystem's
  finddir() for each path component.
- Operations: open, close, read, write, seek, readdir, stat.
- Each filesystem driver provides a vfs_fs_ops_t with callbacks.

Initrd filesystem driver (initrd_fs.c/h):
- Read-only VFS driver backed by the CPIO ramdisk.
- Mounted at '/initrd' during boot.
- Zero-copy reads: file data points directly into the CPIO archive
  memory, no allocation or copying needed.
- Supports readdir (flat iteration) and finddir (name lookup).

Bug fix: resolve_path was overwriting file-specific fs_data (set by
finddir, e.g. pointer to CPIO file data) with the mount's fs_data
(NULL). Fixed to preserve fs_data from finddir.

Verified in QEMU: kernel reads /initrd/README via VFS and prints its
contents. Ring 3 user process continues to work.

src/kernel.c

@@ -14,6 +14,8 @@
 #include "syscall.h"
 #include "process.h"
 #include "cpio.h"
+#include "vfs.h"
+#include "initrd_fs.h"
 
 void offset_print(const char *str)
 {
@@ -101,6 +103,27 @@ void kernel_main(uint32_t magic, uint32_t addr) {
         offset_print("No initrd module found\n");
     }
 
+    init_vfs();
+    offset_print("VFS initialized\n");
+
+    if (initrd_start != 0) {
+        init_initrd_fs();
+        offset_print("Initrd filesystem mounted\n");
+
+        /* Test VFS: read a file from the initrd */
+        int fd = vfs_open("/initrd/README", 0);
+        if (fd >= 0) {
+            char buf[64];
+            int32_t n = vfs_read(fd, buf, sizeof(buf) - 1);
+            if (n > 0) {
+                buf[n] = '\0';
+                offset_print("VFS read /initrd/README: ");
+                offset_print(buf);
+            }
+            vfs_close(fd);
+        }
+    }
+
     init_tss();
     offset_print("TSS initialized\n");