feat: auto-detect max VRAM budget with --max-vram -1 (#1498)

include/stable-diffusion.h

@@ -205,7 +205,7 @@ typedef struct {
     bool chroma_use_t5_mask;
     int chroma_t5_mask_pad;
     bool qwen_image_zero_cond_t;
-    float max_vram;
+    float max_vram;  // GiB budget for graph-cut segmented param offload (0 = disabled, -1 = auto free VRAM minus 1 GiB)
 } sd_ctx_params_t;
 
 typedef struct {