mAi: add 6 style presets — cinematic, watercolor, anime, 3d-render, line-art, isometric

Cross-coordination with flexsiebels/head (paul). m wants more style options
on /imagine/new; flexsiebels has the UI side ready to bump IMAGEN_STYLES
in lib/server/imagen.ts + schemas.ts as soon as the worker accepts them.

styles.yaml: 6 new entries with FLUX-friendly prompt fragments. No code
changes — Apply() and Styles() consume the embedded YAML directly, the
"enum" is dynamic.

prompt_test.go: extend TestStylesContainsAllExpected expectation list
(alphabetical, '3' < 'a' so 3d-render leads).

Total enum: 11 (5 existing + 6 new). flexsiebels delegation message 1669.

internal/prompt/prompt_test.go

@@ -37,7 +37,7 @@ func TestApplyToEmptyPromptUsesPresetOnly(t *testing.T) {
 }
 
 func TestStylesContainsAllExpected(t *testing.T) {
-	want := []string{"blog-header", "diagram", "illustration", "photo", "sketch"}
+	want := []string{"3d-render", "anime", "blog-header", "cinematic", "diagram", "illustration", "isometric", "line-art", "photo", "sketch", "watercolor"}
 	got := Styles()
 	if len(got) != len(want) {
 		t.Fatalf("Styles() = %v, want %v", got, want)

internal/prompt/styles.yaml

@@ -4,3 +4,9 @@ styles:
   diagram:      "minimal technical diagram, isometric, white background, line-art"
   sketch:       "rough pencil sketch, hand-drawn, monochrome"
   blog-header:  "wide aspect, conceptual, soft palette, editorial illustration"
+  cinematic:    "cinematic still, 35mm film, shallow depth of field, dramatic lighting, color graded"
+  watercolor:   "watercolor painting, soft washes, paper texture, loose brushwork"
+  anime:        "anime illustration, cel-shaded, expressive linework, vibrant flat colors"
+  3d-render:    "3d render, octane, soft global illumination, subtle ambient occlusion, physically based materials"
+  line-art:     "clean line art, black ink on white, no shading, even stroke weight"
+  isometric:    "isometric illustration, 30 degree projection, flat colors, crisp geometric shapes"