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

Cargo.toml

@@ -226,3 +226,7 @@ dirs = "6.0.0"
 
 [dev-dependencies]
 tempfile = "3.24.0"
+proptest = "1.6"
+image = { version = "0.25.9", default-features = false, features = ["png"] }
+tiny-skia = "0.11"
+ron = "0.12"