[AI] Add Lua AI API with model inference, tensor ops, and raw CFA support

RELEASE_NOTES.md

@@ -341,7 +341,13 @@ changes (where available).
 
 ### New Features
 
-- N/A
+- Added Lua AI API (`darktable.ai`) for scripting AI model inference.
+  Provides tensor creation, model loading with GPU provider selection,
+  and two calling conventions for inference (pre-allocated and
+  auto-allocated outputs). Image I/O includes loading from file or
+  darktable library (full pipeline export), raw CFA sensor data
+  access, and DNG output with EXIF preservation. Enables Lua scripts
+  to implement custom AI workflows such as raw denoise or upscale.
 
 ### Bug Fixes
 

src/CMakeLists.txt

@@ -792,6 +792,7 @@ if(USE_LUA)
   if(USE_LUA)
     add_definitions("-DUSE_LUA")
     FILE(GLOB SOURCE_FILES_LUA
+      "lua/ai.c"
       "lua/cairo.c"
       "lua/call.c"
       "lua/configuration.c"

src/common/ai_models.c

@@ -951,6 +951,8 @@ void dt_ai_models_cleanup(dt_ai_registry_t *registry)
 
   g_mutex_clear(&registry->lock);
 
+  if(registry->env)
+    dt_ai_env_destroy(registry->env);
   g_free(registry->repository);
   g_free(registry->models_dir);
   g_free(registry->cache_dir);
@@ -1931,6 +1933,19 @@ void dt_ai_model_card_free(dt_ai_model_card_t *card)
   g_free(card);
 }
 
+dt_ai_environment_t *dt_ai_registry_get_env(dt_ai_registry_t *registry)
+{
+  if(!registry || !registry->ai_enabled)
+    return NULL;
+
+  g_mutex_lock(&registry->lock);
+  if(!registry->env)
+    registry->env = dt_ai_env_init(NULL);
+  g_mutex_unlock(&registry->lock);
+
+  return registry->env;
+}
+
 // clang-format off
 // modelines: These editor modelines have been set for all relevant files by tools/update_modelines.py
 // vim: shiftwidth=2 expandtab tabstop=2 cindent

src/common/ai_models.h

@@ -87,6 +87,7 @@ typedef struct dt_ai_registry_t
   gboolean ai_enabled;        // Global AI enable/disable
   dt_ai_provider_t provider;  // Selected execution provider
   gboolean updates_checked;   // TRUE after first check_updates call
+  struct dt_ai_environment_t *env;  // Lazily created backend environment
   GMutex lock;                // Thread safety for registry access
 } dt_ai_registry_t;
 
@@ -348,3 +349,12 @@ dt_ai_model_card_t *dt_ai_models_get_card(
  * @brief Free a model card returned by dt_ai_models_get_card
  */
 void dt_ai_model_card_free(dt_ai_model_card_t *card);
+
+/**
+ * @brief Get or lazily create the AI backend environment.
+ *        The environment is cached in the registry and destroyed
+ *        by dt_ai_models_cleanup().
+ * @param registry The registry
+ * @return Environment handle, or NULL if AI is disabled
+ */
+dt_ai_environment_t *dt_ai_registry_get_env(dt_ai_registry_t *registry);