Improve API safety

This improves API Layer:
- Add runtime NULL checks for all API functions
- Add backend function pointer validation before use
- Graceful error handling in release builds via log_error()
- Fix twin_destroy() to validate context before backend exit

Author: jserv

src/api.c

@@ -22,13 +22,17 @@ extern twin_backend_t g_twin_backend;
  */
 twin_context_t *twin_create(int width, int height)
 {
+    /* Runtime check for missing backend */
+    if (!g_twin_backend.init) {
+        log_error("Backend not registered - no init function");
+        return NULL;
+    }
+
     assert(g_twin_backend.init && "Backend not registered");
 
     twin_context_t *ctx = g_twin_backend.init(width, height);
     if (!ctx) {
-#ifdef CONFIG_LOGGING
-        log_error("Failed to initialize Twin context (%dx%d)", width, height);
-#endif
+        log_error("Backend initialization failed (%dx%d)", width, height);
     }
     return ctx;
 }
@@ -42,10 +46,18 @@ twin_context_t *twin_create(int width, int height)
  */
 void twin_destroy(twin_context_t *ctx)
 {
+    if (!ctx)
+        return;
+
+    /* Runtime check for missing backend */
+    if (!g_twin_backend.exit) {
+        log_error("Backend not registered - no exit function");
+        return;
+    }
+
     assert(g_twin_backend.exit && "Backend not registered");
 
-    if (ctx)
-        g_twin_backend.exit(ctx);
+    g_twin_backend.exit(ctx);
 }
 
 /**
@@ -57,11 +69,24 @@ void twin_destroy(twin_context_t *ctx)
  *
  * @ctx           : Twin context to run
  * @init_callback : Application initialization function (called once before
- * event loop)
+ *                  event loop)
  */
 void twin_run(twin_context_t *ctx, void (*init_callback)(twin_context_t *))
 {
+    /* Validate context parameter */
+    if (!ctx) {
+        log_error("NULL context passed to twin_run");
+        return;
+    }
+
     assert(ctx && "NULL context passed to twin_run");
+
+    /* Runtime check for missing start function */
+    if (!g_twin_backend.start) {
+        log_error("Backend has no start function - main loop not running");
+        return;
+    }
+
     assert(g_twin_backend.start && "Backend start function not registered");
 
     g_twin_backend.start(ctx, init_callback);

src/dispatch.c

@@ -41,9 +41,7 @@ bool twin_dispatch_once(twin_context_t *ctx)
 {
     /* Validate context to prevent null pointer dereference in callbacks */
     if (!ctx) {
-#ifdef CONFIG_LOGGING
         log_error("twin_dispatch_once: NULL context");
-#endif
         return false;
     }
 
@@ -57,9 +55,7 @@ bool twin_dispatch_once(twin_context_t *ctx)
         if (!g_twin_backend.poll(ctx))
             return false;
     } else {
-#ifdef CONFIG_LOGGING
         log_warn("twin_dispatch_once: No backend poll function registered");
-#endif
         /* Yield CPU to avoid busy-waiting when no event source available */
 #ifdef __EMSCRIPTEN__
         emscripten_sleep(0);
@@ -94,18 +90,14 @@ void twin_dispatch(twin_context_t *ctx)
      * Calling twin_dispatch() directly will not work in WebAssembly.
      */
     (void) ctx; /* Unused in Emscripten builds */
-#ifdef CONFIG_LOGGING
     log_error(
         "twin_dispatch() called in Emscripten build - use "
         "twin_dispatch_once() with emscripten_set_main_loop_arg()");
-#endif
     return;
 #else
     /* Validate context before entering event loop */
     if (!ctx) {
-#ifdef CONFIG_LOGGING
         log_error("twin_dispatch: NULL context");
-#endif
         return;
     }