Refactor ruin_app, add some README files, minor usability in reactivity

lib/ruin_app/src/text_style.rs

@@ -0,0 +1,61 @@
+use std::cell::RefCell;
+
+use ruin_ui::{Color, TextFontFamily, TextWrap};
+
+use crate::primitives::FontWeight;
+
+/// Partially-specified text style for context-based inheritance.
+///
+/// Set on a container via `.text_style(...)`. All descendant `text()` nodes inherit
+/// any fields that are `Some`, falling back to their own defaults for fields that are `None`.
+/// Inner (closer) ancestors take precedence over outer ancestors.
+#[derive(Clone, Default)]
+pub struct PartialTextStyle {
+    pub size: Option<f32>,
+    pub color: Option<Color>,
+    pub weight: Option<FontWeight>,
+    pub font_family: Option<TextFontFamily>,
+    pub wrap: Option<TextWrap>,
+    pub line_height: Option<f32>,
+}
+
+impl PartialTextStyle {
+    /// Merge `other` on top of `self` — `other`'s `Some` fields win.
+    fn merge_over(self, other: &PartialTextStyle) -> Self {
+        Self {
+            size: other.size.or(self.size),
+            color: other.color.or(self.color),
+            weight: other.weight.or(self.weight),
+            font_family: other.font_family.clone().or(self.font_family),
+            wrap: other.wrap.or(self.wrap),
+            line_height: other.line_height.or(self.line_height),
+        }
+    }
+}
+
+thread_local! {
+    static TEXT_STYLE_STACK: RefCell<Vec<PartialTextStyle>> = const { RefCell::new(Vec::new()) };
+}
+
+/// Push a partial style onto the context stack. Must be paired with `pop_text_style`.
+pub(crate) fn push_text_style(style: PartialTextStyle) {
+    TEXT_STYLE_STACK.with(|stack| stack.borrow_mut().push(style));
+}
+
+/// Pop the top partial style from the context stack.
+pub(crate) fn pop_text_style() {
+    TEXT_STYLE_STACK.with(|stack| {
+        stack.borrow_mut().pop();
+    });
+}
+
+/// Returns the effective `PartialTextStyle` at the current render position by merging
+/// all layers from outermost to innermost (innermost wins).
+pub(crate) fn current_text_style() -> PartialTextStyle {
+    TEXT_STYLE_STACK.with(|stack| {
+        stack
+            .borrow()
+            .iter()
+            .fold(PartialTextStyle::default(), |acc, layer| acc.merge_over(layer))
+    })
+}