T25a Generative: prompt and settings

PromptEnhancer offers a fuller prompt with the words it adds washed,
split at punctuation; Use this takes it into the field, Keep mine
lets it go. StylePresetPicker and AspectRatioPicker are rows of Tab
stops, the chosen one in accent; a ratio chip draws its shape.
SeedInput keeps digits up to u32::MAX and rolls one with a die.
NegativePromptInput and ImageGenerationPanel point to forms::TagInput
and forms::Form. theme.generative() holds the chapter's sizes. The
gallery's preset pictures are the atrium photo in five styles.

src/generative/mod.rs

@@ -0,0 +1,11 @@
+mod prompt;
+mod ratio;
+mod seed;
+mod style;
+#[cfg(all(test, feature = "test-support"))]
+mod tests;
+
+pub use prompt::PromptEnhancer;
+pub use ratio::AspectRatioPicker;
+pub use seed::SeedInput;
+pub use style::{StylePreset, StylePresetPicker};