feat: add configurable conditioning cache for all models (#2034)

docs/caching.md

@@ -2,6 +2,16 @@
 
 Caching methods accelerate diffusion inference by reusing intermediate computations when changes between steps are small.
 
+### Conditioning Cache
+
+Conditioning results are cached per model context using an LRU cache. The default
+capacity is **0 (disabled) for `sd-cli`** and **4 entries for `sd-server` and the C
+API**. Set `--conditioning-cache-size N` to change the limit; `0` disables caching.
+For example, `sd-cli -m model.safetensors -p "a cat" --conditioning-cache-size 4`
+enables the cache in the CLI. The C API option is
+`sd_ctx_params_t::conditioning_cache_size`, initialized by `sd_ctx_params_init()`.
+This cache is independent of the diffusion-step `--cache-mode` options below.
+
 ### Cache Modes
 
 | Mode | Target | Description |