Add sysfs VFS driver, SYS_OPEN/CLOSE syscalls, cat app

Sysfs:
- New VFS driver mounted at /sys that lets kernel drivers expose
  virtual text files via namespace registration
- Drivers call sysfs_register(name, ops, ctx) with list/read/write
  callbacks for their namespace
- IDE driver registers 'ide' namespace exposing per-device attributes:
  model, type, channel, drive, sectors, sector_size
- Tested: ls /sys -> ide, ls /sys/ide -> hdd1 cd1,
  cat /sys/ide/hdd1/model -> QEMU HARDDISK

Syscalls:
- Added SYS_OPEN (11) and SYS_CLOSE (12) for file I/O from userspace
- Extended SYS_READ/SYS_WRITE to handle VFS file descriptors (fd >= 3)
- Updated userspace syscalls.h with open()/close() wrappers

Apps:
- New 'cat' app: reads and displays file contents via open/read/close
- Updated 'ls' to accept path argument via ARG1 env var
- Updated shell to pass ARG1 env var to external commands

apps/libc/syscalls.h

@@ -24,6 +24,8 @@ typedef int int32_t;
 #define SYS_GETENV  8
 #define SYS_SETENV  9
 #define SYS_READDIR 10
+#define SYS_OPEN    11
+#define SYS_CLOSE   12
 
 static inline int32_t syscall0(int num) {
     int32_t ret;
@@ -103,6 +105,25 @@ static inline int32_t readdir(const char *path, uint32_t idx, char *name) {
     return syscall3(SYS_READDIR, (uint32_t)path, idx, (uint32_t)name);
 }
 
+/**
+ * Open a file by path.
+ * @param path  File path.
+ * @param flags Open flags (currently unused, pass 0).
+ * @return File descriptor (>= 3) on success, -1 on failure.
+ */
+static inline int32_t open(const char *path, uint32_t flags) {
+    return syscall2(SYS_OPEN, (uint32_t)path, flags);
+}
+
+/**
+ * Close a file descriptor.
+ * @param fd File descriptor.
+ * @return 0 on success, -1 on failure.
+ */
+static inline int32_t close(int32_t fd) {
+    return syscall1(SYS_CLOSE, (uint32_t)fd);
+}
+
 /* Basic string operations for user-space */
 static inline uint32_t strlen(const char *s) {
     uint32_t len = 0;