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.

imagegen/imagegen.go

@@ -38,6 +38,29 @@ type Request struct {
 	// Size is the requested resolution, e.g. "512x512" or "1024x1024";
 	// "" = provider default.
 	Size string
+
+	// The fields below are optional per-request overrides. Their zero value
+	// (nil pointer or empty string) means "leave the backend's own default" —
+	// for stable-diffusion.cpp that is the per-model default baked into the
+	// llama-swap launch flags. A caller overrides only what it explicitly sets.
+
+	// Steps is the number of diffusion steps; nil = backend default.
+	Steps *int
+
+	// CFGScale is the classifier-free-guidance scale; nil = backend default.
+	// Architecture-sensitive (SDXL likes ~7, Flux wants 1), so prefer leaving
+	// it nil unless the caller knows the target model.
+	CFGScale *float64
+
+	// NegativePrompt steers generation away from concepts; "" = none.
+	NegativePrompt string
+
+	// Sampler selects the sampling method (e.g. "euler", "euler_a");
+	// "" = backend default.
+	Sampler string
+
+	// Seed fixes the RNG seed for reproducible output; nil = random.
+	Seed *int64
 }
 
 // Result is the canonical image-generation result.
@@ -60,6 +83,21 @@ func WithN(n int) Option { return func(r *Request) { r.N = n } }
 // WithSize sets the requested resolution (e.g. "1024x1024").
 func WithSize(size string) Option { return func(r *Request) { r.Size = size } }
 
+// WithSteps overrides the number of diffusion steps.
+func WithSteps(n int) Option { return func(r *Request) { r.Steps = &n } }
+
+// WithCFGScale overrides the classifier-free-guidance scale.
+func WithCFGScale(s float64) Option { return func(r *Request) { r.CFGScale = &s } }
+
+// WithNegativePrompt sets a negative prompt.
+func WithNegativePrompt(s string) Option { return func(r *Request) { r.NegativePrompt = s } }
+
+// WithSampler overrides the sampling method (e.g. "euler", "euler_a").
+func WithSampler(s string) Option { return func(r *Request) { r.Sampler = s } }
+
+// WithSeed fixes the RNG seed for reproducible output.
+func WithSeed(seed int64) Option { return func(r *Request) { r.Seed = &seed } }
+
 // Apply returns a copy of the request with all options applied. Providers call
 // this once at the top of Generate.
 func (r Request) Apply(opts ...Option) Request {