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

src/kernel.c

@@ -20,6 +20,7 @@
 #include "devicefs.h"
 #include "sysfs.h"
 #include "mbr.h"
+#include "fat32.h"
 #include "keyboard.h"
 #include "framebuffer.h"
 
@@ -68,6 +69,128 @@ void print_hex(uint32_t val)
     outb(0xE9, '\n'); serial_putc('\n');
 }
 
+/* ================================================================
+ * VFS sysfs namespace — handles mount/umount via /sys/vfs
+ * ================================================================
+ *
+ * /sys/vfs/mount  — write "device mountpoint" to mount a FAT32 device
+ * /sys/vfs/mounts — read to list active mounts
+ */
+
+/** Last mount status message for reading back. */
+static char vfs_sysfs_status[256] = "no mounts\n";
+
+static int vfs_sysfs_list(void *ctx, const char *path, uint32_t idx,
+                           sysfs_entry_t *out) {
+    (void)ctx;
+    if (path[0] != '\0') return -1;  /* No subdirectories */
+
+    static const char *entries[] = { "mount", "mounts" };
+    if (idx >= 2) return -1;
+
+    memset(out, 0, sizeof(sysfs_entry_t));
+    strncpy(out->name, entries[idx], SYSFS_MAX_NAME - 1);
+    out->is_dir = 0;
+    return 0;
+}
+
+static int vfs_sysfs_read(void *ctx, const char *path, char *buf,
+                           uint32_t buf_size) {
+    (void)ctx;
+
+    if (strcmp(path, "mount") == 0 || strcmp(path, "mounts") == 0) {
+        uint32_t len = strlen(vfs_sysfs_status);
+        if (len + 1 > buf_size) len = buf_size - 1;
+        memcpy(buf, vfs_sysfs_status, len);
+        buf[len] = '\0';
+        return (int)len;
+    }
+    return -1;
+}
+
+static int vfs_sysfs_write(void *ctx, const char *path, const char *buf,
+                            uint32_t size) {
+    (void)ctx;
+
+    if (strcmp(path, "mount") != 0) return -1;
+
+    /* Parse: "device_name mount_path\n" */
+    char device[64];
+    char mount_path[128];
+    int di = 0, mi = 0;
+    uint32_t i = 0;
+
+    /* Parse device name */
+    while (i < size && buf[i] != ' ' && buf[i] != '\n' && di < 63) {
+        device[di++] = buf[i++];
+    }
+    device[di] = '\0';
+
+    /* Skip spaces */
+    while (i < size && buf[i] == ' ') i++;
+
+    /* Parse mount path */
+    while (i < size && buf[i] != ' ' && buf[i] != '\n' && mi < 127) {
+        mount_path[mi++] = buf[i++];
+    }
+    mount_path[mi] = '\0';
+
+    if (di == 0 || mi == 0) {
+        strncpy(vfs_sysfs_status, "error: usage: device mountpoint\n",
+                sizeof(vfs_sysfs_status) - 1);
+        return -1;
+    }
+
+    offset_print("  VFS mount request: ");
+    offset_print(device);
+    offset_print(" -> ");
+    offset_print(mount_path);
+    offset_print("\n");
+
+    /* Find the device */
+    devicefs_device_t *dev = devicefs_find(device);
+    if (!dev) {
+        strncpy(vfs_sysfs_status, "error: device not found\n",
+                sizeof(vfs_sysfs_status) - 1);
+        offset_print("  VFS mount: device not found\n");
+        return -1;
+    }
+
+    /* Mount as FAT32 */
+    int ret = fat32_mount_device(dev, mount_path);
+    if (ret != 0) {
+        strncpy(vfs_sysfs_status, "error: FAT32 mount failed\n",
+                sizeof(vfs_sysfs_status) - 1);
+        offset_print("  VFS mount: FAT32 mount failed\n");
+        return -1;
+    }
+
+    /* Build success message */
+    strncpy(vfs_sysfs_status, "mounted ", sizeof(vfs_sysfs_status) - 1);
+    uint32_t slen = strlen(vfs_sysfs_status);
+    strncpy(vfs_sysfs_status + slen, device,
+            sizeof(vfs_sysfs_status) - 1 - slen);
+    slen = strlen(vfs_sysfs_status);
+    strncpy(vfs_sysfs_status + slen, " at ",
+            sizeof(vfs_sysfs_status) - 1 - slen);
+    slen = strlen(vfs_sysfs_status);
+    strncpy(vfs_sysfs_status + slen, mount_path,
+            sizeof(vfs_sysfs_status) - 1 - slen);
+    slen = strlen(vfs_sysfs_status);
+    if (slen < sizeof(vfs_sysfs_status) - 1) {
+        vfs_sysfs_status[slen] = '\n';
+        vfs_sysfs_status[slen + 1] = '\0';
+    }
+
+    return (int)size;
+}
+
+static sysfs_ops_t vfs_sysfs_ops = {
+    .list  = vfs_sysfs_list,
+    .read  = vfs_sysfs_read,
+    .write = vfs_sysfs_write,
+};
+
 void kernel_main(uint32_t magic, uint32_t addr) {
     /* Initialize serial port first so all debug output goes to COM1 too */
     serial_init();
@@ -258,6 +381,10 @@ void kernel_main(uint32_t magic, uint32_t addr) {
     EARLY_PRINT("SYS ");
     offset_print("Sysfs initialized\n");
 
+    /* Register VFS mount namespace in sysfs */
+    sysfs_register("vfs", &vfs_sysfs_ops, NULL);
+    offset_print("VFS sysfs namespace registered\n");
+
     init_tss();
     EARLY_PRINT("TSS ");
     offset_print("TSS initialized\n");