Add comprehensive testing infrastructure: visual snapshots, unit, property, and theme tests

Introduces a multi-layered testing strategy for the libcosmic widget toolkit:

- Visual snapshot testing: Renders widgets via tiny-skia software renderer and
  compares pixel output against reference PNGs. Generates diff images on failure.
  Covers containers, dividers, text styles, progress bars, and layouts.

- Widget unit tests: Thorough behavioral tests for segmented button model
  (selection, ordering, data storage, enable/disable, closable, indent),
  calendar model (month navigation, year wrapping, date selection, grid layout),
  spin button (clamping, increment/decrement, float support), and color picker.

- Property-based tests (proptest): Randomized input testing for spin button
  bounds invariants, calendar first-day-of-week correctness, segmented model
  length/clear/toggle invariants, and theme spacing monotonicity.

- Theme consistency tests: Validates all four theme variants (dark, light,
  high-contrast dark/light) have valid spacing, corner radii ordering,
  non-zero-alpha colors, visible text-on-background, and RON round-trip
  serialization.

- Expanded inline tests: 16 new spin_button unit tests covering increment,
  decrement, clamping, float types, and constructor value bounds.

- CI integration: New visual-tests job runs snapshot, theme, property, and
  widget tests with artifact upload on failure for snapshot diffs.

https://claude.ai/code/session_01P3WLXMif9wmNXbWDo5FhRQ

tests/visual_tests.rs

@@ -0,0 +1,183 @@
+// Copyright 2024 System76 <info@system76.com>
+// SPDX-License-Identifier: MPL-2.0
+
+//! Visual snapshot tests for COSMIC widgets.
+//!
+//! These tests render widgets using the tiny-skia software renderer and compare
+//! the output against stored reference snapshots. This catches unintended visual
+//! regressions in widget rendering, theming, and layout.
+//!
+//! # Updating Snapshots
+//!
+//! When a visual change is intentional, delete the old snapshot PNG from
+//! `tests/snapshots/` and re-run the tests. The new reference will be generated
+//! automatically.
+
+mod snapshot_harness;
+
+use cosmic::iced_core::Length;
+use cosmic::theme;
+use snapshot_harness::SnapshotHarness;
+
+/// Helper to render a cosmic Element to a snapshot.
+fn render_element_snapshot(
+    name: &str,
+    width: u32,
+    height: u32,
+    element: cosmic::Element<'_, ()>,
+) {
+    let harness = SnapshotHarness::new(width, height);
+    harness.snapshot(name, element);
+}
+
+// ---------------------------------------------------------------------------
+// Theme rendering tests - verify that the cosmic theme system produces
+// consistent visual output for core UI primitives.
+// ---------------------------------------------------------------------------
+
+#[test]
+fn snapshot_container_with_background() {
+    use cosmic::widget::{container, text};
+
+    let content = text::body("Hello, COSMIC!")
+        .apply(container)
+        .padding(16)
+        .width(Length::Fixed(200.0))
+        .height(Length::Fixed(60.0))
+        .class(theme::Container::Primary);
+
+    render_element_snapshot("container_with_background", 220, 80, content.into());
+}
+
+#[test]
+fn snapshot_divider_horizontal() {
+    use cosmic::widget::{column, container, divider};
+
+    let content = column::with_capacity(3)
+        .push(cosmic::widget::text::body("Above"))
+        .push(divider::horizontal::default())
+        .push(cosmic::widget::text::body("Below"))
+        .width(Length::Fixed(200.0))
+        .apply(container)
+        .padding(8);
+
+    render_element_snapshot("divider_horizontal", 220, 100, content.into());
+}
+
+#[test]
+fn snapshot_divider_heavy() {
+    use cosmic::widget::{column, container, divider};
+
+    let content = column::with_capacity(3)
+        .push(cosmic::widget::text::body("Above"))
+        .push(divider::horizontal::heavy())
+        .push(cosmic::widget::text::body("Below"))
+        .width(Length::Fixed(200.0))
+        .apply(container)
+        .padding(8);
+
+    render_element_snapshot("divider_heavy", 220, 100, content.into());
+}
+
+#[test]
+fn snapshot_text_styles() {
+    use cosmic::widget::{column, container, text};
+
+    let content = column::with_capacity(4)
+        .push(text::title3("Title Text"))
+        .push(text::body("Body text content"))
+        .push(text::caption("Caption text"))
+        .push(text::monotext("Monospace text"))
+        .spacing(8)
+        .width(Length::Fixed(300.0))
+        .apply(container)
+        .padding(16);
+
+    render_element_snapshot("text_styles", 340, 200, content.into());
+}
+
+#[test]
+fn snapshot_progress_bar() {
+    use cosmic::widget::{column, container, progress_bar};
+
+    let content = column::with_capacity(3)
+        .push(progress_bar(0.0..=100.0, 0.0).width(Length::Fixed(250.0)))
+        .push(progress_bar(0.0..=100.0, 50.0).width(Length::Fixed(250.0)))
+        .push(progress_bar(0.0..=100.0, 100.0).width(Length::Fixed(250.0)))
+        .spacing(12)
+        .apply(container)
+        .padding(16);
+
+    render_element_snapshot("progress_bar_states", 290, 120, content.into());
+}
+
+#[test]
+fn snapshot_row_layout() {
+    use cosmic::widget::{container, row, text};
+
+    let content = row::with_capacity(3)
+        .push(text::body("Left"))
+        .push(text::body("Center"))
+        .push(text::body("Right"))
+        .spacing(16)
+        .apply(container)
+        .padding(12);
+
+    render_element_snapshot("row_layout", 300, 60, content.into());
+}
+
+#[test]
+fn snapshot_column_layout() {
+    use cosmic::widget::{column, container, text};
+
+    let content = column::with_capacity(3)
+        .push(text::body("First"))
+        .push(text::body("Second"))
+        .push(text::body("Third"))
+        .spacing(8)
+        .apply(container)
+        .padding(12);
+
+    render_element_snapshot("column_layout", 200, 120, content.into());
+}
+
+#[test]
+fn snapshot_nested_containers() {
+    use cosmic::widget::{column, container, row, text};
+
+    let inner = column::with_capacity(2)
+        .push(text::body("Nested"))
+        .push(text::caption("Content"))
+        .spacing(4)
+        .apply(container)
+        .padding(8)
+        .class(theme::Container::Primary);
+
+    let outer = row::with_capacity(2)
+        .push(inner)
+        .push(text::body("Adjacent"))
+        .spacing(16)
+        .apply(container)
+        .padding(12);
+
+    render_element_snapshot("nested_containers", 300, 120, outer.into());
+}
+
+#[test]
+fn snapshot_dark_theme_container() {
+    use cosmic::widget::{container, text};
+
+    // Set dark theme temporarily for this test
+    let content = text::body("Dark Theme")
+        .apply(container)
+        .padding(16)
+        .width(Length::Fixed(200.0))
+        .height(Length::Fixed(60.0))
+        .class(theme::Container::Primary);
+
+    // The default theme is dark, so this tests the default rendering
+    render_element_snapshot("dark_theme_container", 220, 80, content.into());
+}
+
+// We use `apply` from the Apply trait for chaining
+use apply::Apply;