Implement simulation client for deterministic testing

Added a simulation client that can run inside the TinyDFS simulation
framework, enabling fully deterministic end-to-end testing of the
distributed file system.

**New Files:**
- src/simulation_client.h - Header defining SimulationClient structure
  and API (init, step, free functions)
- src/simulation_client.c - Implementation of simulation client with
  test operations (create dir, create file, write, read, list, delete)

**Key Changes:**

**System Framework (src/system.c):**
- Added ProcessType enum (METADATA_SERVER, CHUNK_SERVER, CLIENT)
  to distinguish different simulated process types
- Extended Process union to include SimulationClient
- Modified spawn_simulated_process() to detect --client flag and
  initialize client processes using simulation_client_init()
- Updated update_simulation() to call simulation_client_step() for
  client processes, handling timeout management
- Fixed cleanup_simulation() to properly set current_process before
  freeing, preventing NULL pointer dereferences in mock_close()
- Added simulated_time static variable (struct timespec) for
  deterministic clock mocking
- Added helper function is_client() to detect client processes

**API Updates (src/system.h):**
- Exported startup_simulation() function for proper initialization
- Added function declarations for simulation management

**Test Updates (src/main_test.c):**
- Added startup_simulation() call for proper initialization
- Spawns simulation client process with "--client" flag
- Increased iteration limit to 100,000 to allow operations to complete
- Added progress logging every 10,000 iterations
- Added explicit error checking and debug output

**Simulation Client Features:**
- Non-blocking operation model using tinydfs_isdone() and
  tinydfs_process_events()
- State machine for sequential test operations
- Comprehensive test scenario:
  1. Create directory (/test_dir)
  2. Create file (/test_dir/test_file.txt)
  3. Write data to file
  4. Read data back
  5. List directory contents
  6. Delete file
- Proper integration with TinyDFS client library
- Returns poll descriptors from tinydfs_process_events()
- Timeout management for simulation scheduling

**Bug Fixes:**
- Fixed missing return statement in spawn_simulated_process()
- Fixed NULL pointer dereference in cleanup by setting current_process
- Added missing stdlib.h include for atoi()

The simulation client successfully initializes and integrates into the
simulation loop, demonstrating the framework's ability to run client
code deterministically alongside servers.

src/simulation_client.h

@@ -0,0 +1,46 @@
+#ifndef SIMULATION_CLIENT_INCLUDED
+#define SIMULATION_CLIENT_INCLUDED
+
+#include <stdint.h>
+#include <stdbool.h>
+
+#ifdef _WIN32
+#include <winsock2.h>
+#else
+#include <poll.h>
+#endif
+
+#include "TinyDFS.h"
+
+typedef enum {
+    CLIENT_STATE_INIT,
+    CLIENT_STATE_RUNNING,
+    CLIENT_STATE_DONE,
+} ClientState;
+
+typedef struct {
+    TinyDFS *tdfs;
+    ClientState state;
+
+    // Track operations
+    int create_dir_op;
+    int create_file_op;
+    int write_op;
+    int read_op;
+    int list_op;
+    int delete_op;
+
+    // Read buffer
+    char read_buffer[1024];
+
+    // Test step counter
+    int step;
+} SimulationClient;
+
+int  simulation_client_init(SimulationClient *client, int argc, char **argv,
+                            void **contexts, struct pollfd *polled, int *timeout);
+int  simulation_client_step(SimulationClient *client, void **contexts,
+                            struct pollfd *polled, int num_polled, int *timeout);
+void simulation_client_free(SimulationClient *client);
+
+#endif // SIMULATION_CLIENT_INCLUDED