feat(imagegen): optional per-request generation settings

Add Steps, CFGScale, NegativePrompt, Sampler, Seed to imagegen.Request
(pointer/empty = leave the backend's per-model default), with mirror
options, and forward them in the llamaswap wire payload as the
stable-diffusion.cpp fields (steps/cfg_scale/negative_prompt/
sample_method/seed). Unset fields are omitted so sd-server keeps its
baked defaults.

Lets callers (e.g. mort drawbots) override only what they explicitly set.

provider/llamaswap/image.go

@@ -27,14 +27,23 @@ type imageModel struct {
 	id string
 }
 
-// imageRequest is the OpenAI /v1/images/generations request shape. We always
-// request b64_json so the bytes come back inline (no second fetch).
+// imageRequest is the OpenAI /v1/images/generations request shape, plus the
+// stable-diffusion.cpp extras llama-swap forwards to sd-server. We always
+// request b64_json so the bytes come back inline (no second fetch). The
+// optional fields are pointers/omitempty so an unset value is omitted entirely
+// and sd-server falls back to the model's own default (a field name a given
+// sd-server build doesn't recognize is simply ignored — harmless).
 type imageRequest struct {
-	Model          string `json:"model"`
-	Prompt         string `json:"prompt"`
-	N              int    `json:"n,omitempty"`
-	Size           string `json:"size,omitempty"`
-	ResponseFormat string `json:"response_format"`
+	Model          string   `json:"model"`
+	Prompt         string   `json:"prompt"`
+	N              int      `json:"n,omitempty"`
+	Size           string   `json:"size,omitempty"`
+	ResponseFormat string   `json:"response_format"`
+	Steps          *int     `json:"steps,omitempty"`
+	CFGScale       *float64 `json:"cfg_scale,omitempty"`
+	NegativePrompt string   `json:"negative_prompt,omitempty"`
+	SampleMethod   string   `json:"sample_method,omitempty"`
+	Seed           *int64   `json:"seed,omitempty"`
 }
 
 type imageResponse struct {
@@ -61,6 +70,11 @@ func (m *imageModel) Generate(ctx context.Context, req imagegen.Request, opts ..
 		N:              req.N,
 		Size:           req.Size,
 		ResponseFormat: "b64_json",
+		Steps:          req.Steps,
+		CFGScale:       req.CFGScale,
+		NegativePrompt: req.NegativePrompt,
+		SampleMethod:   req.Sampler,
+		Seed:           req.Seed,
 	}
 
 	var resp imageResponse