Add FAT32 driver, mount app, fix shell cd/path resolution

- FAT32 VFS driver (fat32.h, fat32.c): reads BPB, FAT table, cluster
  chains, directory entries (8.3 SFN), file read/write, mount via
  fat32_mount_device()
- mount app: writes 'device mountpoint' to /sys/vfs/mount to trigger
  FAT32 mount from userspace
- VFS sysfs mount namespace in kernel.c: handles mount requests via
  sysfs write to /sys/vfs/mount, delegates to FAT32 driver
- Shell cd: validates target directory exists before updating CWD,
  resolves relative paths (., .., components) to absolute paths
- Shell run_command: resolves ARG1 paths relative to CWD so apps
  like cat and ls receive absolute paths automatically
- Fixed FAT32 root directory access: use fs->root_cluster when
  VFS mount root node has inode=0

apps/mount/mount.c

@@ -0,0 +1,95 @@
+/**
+ * @file mount.c
+ * @brief Mount a block device as FAT32.
+ *
+ * Writes to /sys/vfs/mount to trigger a kernel-level FAT32 mount.
+ *
+ * Usage: mount <device> <path>
+ *   e.g.: mount hdd1mbr1 /mnt
+ */
+
+#include "syscalls.h"
+
+int main(void) {
+    char device[64];
+    char path[128];
+
+    if (getenv("ARG1", device, sizeof(device)) < 0 || device[0] == '\0') {
+        puts("Usage: mount <device> <mountpoint>\n");
+        puts("  e.g.: mount hdd1mbr1 /mnt\n");
+        return 1;
+    }
+
+    /* ARG1 contains "device path" — we need to split it.
+     * Actually, the shell currently only passes the first word as ARG1.
+     * Let's accept: mount <device> <path> where ARG1 = "device path" */
+
+    /* Find the space separating device and path */
+    int i = 0;
+    while (device[i] && device[i] != ' ') i++;
+
+    if (device[i] == ' ') {
+        /* ARG1 contains both: "hdd1mbr1 /mnt" */
+        device[i] = '\0';
+        /* Copy path part */
+        int j = i + 1;
+        while (device[j] == ' ') j++;
+        int pi = 0;
+        while (device[j] && pi < 127) {
+            path[pi++] = device[j++];
+        }
+        path[pi] = '\0';
+    } else {
+        /* Only device, no path — default to /mnt */
+        path[0] = '/'; path[1] = 'm'; path[2] = 'n'; path[3] = 't';
+        path[4] = '\0';
+    }
+
+    if (path[0] == '\0') {
+        puts("mount: missing mountpoint, using /mnt\n");
+        path[0] = '/'; path[1] = 'm'; path[2] = 'n'; path[3] = 't';
+        path[4] = '\0';
+    }
+
+    /* Build the mount command: "device path" */
+    char cmd[192];
+    int pos = 0;
+    for (int k = 0; device[k] && pos < 190; k++) cmd[pos++] = device[k];
+    cmd[pos++] = ' ';
+    for (int k = 0; path[k] && pos < 190; k++) cmd[pos++] = path[k];
+    cmd[pos] = '\0';
+
+    /* Open /sys/vfs/mount and write the command */
+    int32_t fd = open("/sys/vfs/mount", 0);
+    if (fd < 0) {
+        puts("mount: cannot open /sys/vfs/mount\n");
+        return 1;
+    }
+
+    int32_t n = write(fd, cmd, (uint32_t)pos);
+    close(fd);
+
+    if (n < 0) {
+        /* Read the status to show the error */
+        fd = open("/sys/vfs/mounts", 0);
+        if (fd >= 0) {
+            char status[256];
+            int32_t sn = read(fd, status, sizeof(status) - 1);
+            close(fd);
+            if (sn > 0) {
+                status[sn] = '\0';
+                puts(status);
+            }
+        } else {
+            puts("mount: failed\n");
+        }
+        return 1;
+    }
+
+    puts("mount: ");
+    puts(device);
+    puts(" mounted at ");
+    puts(path);
+    puts("\n");
+    return 0;
+}